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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.

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Biochemistry. 5th edition.

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Section 16.1Glycolysis Is an Energy-Conversion Pathway in Many Organisms

We now start our consideration of the glycolytic pathway. This pathway is common to virtually all cells, both prokaryotic and eukaryotic. In eukaryotic cells, glycolysis takes place in the cytosol. This pathway can be thought of as comprising three stages (Figure 16.3). Stage 1, which is the conversion of glucose into fructose 1,6-bisphosphate, consists of three steps: a phosphorylation, an isomerization, and a second phosphorylation reaction. The strategy of these initial steps in glycolysis is to trap the glucose in the cell and form a compound that can be readily cleaved into phosphorylated three-carbon units. Stage 2 is the cleavage of the fructose 1,6-bisphosphate into two three-carbon fragments. These resulting three-carbon units are readily interconvertible. In stage 3, ATP is harvested when the three-carbon fragments are oxidized to pyruvate.

Figure 16.3. Stages of Glycolysis.

Figure 16.3

Stages of Glycolysis. The glycolytic pathway can be divided into three stages: (1) glucose is trapped and destabilized; (2) two interconvertible three-carbon molecules are generated by cleavage of six-carbon fructose; and (3) ATP is generated.

16.1.1. Hexokinase Traps Glucose in the Cell and Begins Glycolysis

Glucose enters cells through specific transport proteins (Section 16.2.4) and has one principal fate: it is phosphorylated by ATP to form glucose 6-phosphate. This step is notable for two reasons: (1) glucose 6-phosphate cannot diffuse through the membrane, because of its negative charges, and (2) the addition of the phosphoryl group begins to destabilize glucose, thus facilitating its further metabolism. The transfer of the phosphoryl group from ATP to the hydroxyl group on carbon 6 of glucose is catalyzed by hexokinase.

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Stage 1 of glycolysis.

Figure

Stage 1 of glycolysis. The three steps of stage 1 begin with the phosphorylation of glucose by hexokinase.

Phosphoryl transfer is a fundamental reaction in biochemistry and is one that was discussed in mechanistic and structural detail earlier (Section 9.4). Kinases are enzymes that catalyze the transfer of a phosphoryl group from ATP to an acceptor. Hexokinase, then, catalyzes the transfer of a phosphoryl group from ATP to a variety of six-carbon sugars (hexoses), such as glucose and mannose. Hexokinase, like adenylate kinase (Section 9.4.2) and all other kinases, requires Mg2+ (or another divalent metal ion such as Mn2+) for activity. The divalent metal ion forms a complex with ATP.

The results of x-ray crystallographic studies of yeast hexokinase revealed that the binding of glucose induces a large conformational change in the enzyme, analogous to the conformational changes undergone by NMP kinases on substrate binding (Section 9.4.3). Hexokinase consists of two lobes, which move toward each other when glucose is bound (Figure 16.4). On glucose binding, one lobe rotates 12 degrees with respect to the other, resulting in movements of the polypeptide backbone of as much as 8 Å. The cleft between the lobes closes, and the bound glucose becomes surrounded by protein, except for the hydroxyl group of carbon 6, which will accept the phosphoryl group from ATP. The closing of the cleft in hexokinase is a striking example of the role of induced fit in enzyme action (Section 8.3.2).

Figure 16.4. Induced Fit in Hexokinase.

Figure 16.4

Induced Fit in Hexokinase. Image mouse.jpg As shown in blue, the two lobes of hexokinase are separated in the absence of glucose. The conformation of hexokinase changes markedly on binding glucose, as shown in red. The two lobes of the enzyme come together and surround (more...)

The glucose-induced structural changes are significant in two respects. First, the environment around the glucose becomes much more nonpolar, which favors the donation of the terminal phosphoryl group of ATP. Second, as noted in Section 9.4.3, the substrate-induced conformational changes within the kinase enables it to discriminate against H2O as a substrate. If hexokinase were rigid, a molecule of H2O occupying the binding site for the-CH2OH of glucose would attack the γ phosphoryl group of ATP, forming ADP and Pi. In other words, a rigid kinase would necessarily also be an ATPase. It is interesting to note that other kinases taking part in glycolysis—pyruvate kinase, phosphoglycerate kinase, and phosphofructokinase—also contain clefts between lobes that close when substrate is bound, although the structures of these enzymes are different in other regards. Substrate-induced cleft closing is a general feature of kinases.

16.1.2. The Formation of Fructose 1,6-bisphosphate from Glucose 6-phosphate

The next step in glycolysis is the isomerization of glucose 6-phosphate to fructose 6-phosphate. Recall that the open-chain form of glucose has an aldehyde group at carbon 1, whereas the open-chain form of fructose has a keto group at carbon 2. Thus, the isomerization of glucose 6-phosphate to fructose 6-phosphate is a conversion of an aldose into a ketose. The reaction catalyzed by phosphoglucose isomerase includes additional steps because both glucose 6-phosphate and fructose 6-phosphate are present primarily in the cyclic forms. The enzyme must first open the six-membered ring of glucose 6-phosphate, catalyze the isomerization, and then promote the formation of the five-membered ring of fructose 6-phosphate.

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A second phosphorylation reaction follows the isomerization step. Fructose 6-phosphate is phosphorylated by ATP to fructose 1,6-bisphosphate (F-1,6-BP). The prefix bis- in bisphosphate means that two separate monophosphate groups are present, whereas the prefix di- in diphosphate (as in adenosine diphosphate) means that two phosphate groups are present and are connected by an anhydride bond.

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This reaction is catalyzed by phosphofructokinase (PFK), an allosteric enzyme that sets the pace of glycolysis (Section 16.2.1). As we will learn, this enzyme plays a central role in the integration of much of metabolism.

16.1.3. The Six-Carbon Sugar Is Cleaved into Two Three-Carbon Fragments by Aldolase

The second stage of glycolysis begins with the splitting of fructose 1,6-bisphosphate into glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). The products of the remaining steps in glycolysis consist of three-carbon units rather than six-carbon units.

Image ch16fu6.jpg

Stage 2 of glycolysis.

Figure

Stage 2 of glycolysis. Two three-carbon fragments are produced from one six-carbon sugar.

This reaction is catalyzed by aldolase. This enzyme derives its name from the nature of the reverse reaction, an aldol condensation. The reaction catalyzed by aldolase is readily reversible under intracellular conditions.

16.1.4. Triose phosphate isomerase Salvages a Three-Carbon Fragment

Glyceraldehyde 3-phosphate is on the direct pathway of glycolysis, whereas dihydroxyacetone phosphate is not. Unless a means exists to convert dihydroxyacetone phosphate into glyceraldehyde 3-phosphate, a three-carbon fragment useful for generating ATP will be lost. These compounds are isomers that can be readily interconverted: dihydroxyacetone phosphate is a ketose, whereas glyceraldehyde 3-phosphate is an aldose. The isomerization of these three-carbon phosphorylated sugars is catalyzed by triose phosphate isomerase (TIM; Figure 16.5). This reaction is rapid and reversible. At equilibrium, 96% of the triose phosphate is dihydroxyacetone phosphate. However, the reaction proceeds readily from dihydroxyacetone phosphate to glyceraldehyde 3-phosphate because the subsequent reactions of glycolysis remove this product.

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Figure 16.5. Structure of Triose Phosphate Isomerase.

Figure 16.5

Structure of Triose Phosphate Isomerase. Image mouse.jpg This enzyme consists of a central core of eight parallel β strands (orange) surrounded by eight α helices (blue). This structural motif, called an αβ barrel, is also found in the (more...)

Much is known about the catalytic mechanism of triose phosphate isomerase. TIM catalyzes the transfer of a hydrogen atom from carbon 1 to carbon 2 in converting dihydroxyacetone phosphate into glyceraldehyde 3-phosphate, an intramolecular oxidation-reduction. This isomerization of a ketose into an aldose proceeds through an enediol intermediate (Figure 16.6).

Figure 16.6. Catalytic Mechanism of Triose Phosphate Isomerase.

Figure 16.6

Catalytic Mechanism of Triose Phosphate Isomerase. Glutamate 165 transfers a proton between carbons with the assistance of histidine 95, which shuttles between the neutral and relatively rare negatively charged form. The latter is stabilized by interactions (more...)

X-ray crystallographic and other studies showed that glutamate 165 (see Figure 16.5) plays the role of a general acid-base catalyst. However, this carboxylate group by itself is not basic enough to pull a proton away from a carbon atom adjacent to a carbonyl group. Histidine 95 assists catalysis by donating a proton to stabilize the negative charge that develops on the C-2 carbonyl group.

Two features of this enzyme are noteworthy. First, TIM displays great catalytic prowess. It accelerates isomerization by a factor of 1010 compared with the rate obtained with a simple base catalyst such as acetate ion. Indeed, the kcat/KM ratio for isomerization of glyceraldehyde 3-phosphate is 2 × 108 M-1 s-1, which is close to the diffusion-controlled limit. In other words, the rate-limiting step in catalysis is the diffusion-controlled encounter of substrate and enzyme. TIM is an example of a kinetically perfect enzyme (Section 8.2.5). Second, TIM suppresses an undesired side reaction, the decomposition of the enediol intermediate into methyl glyoxal and inorganic phosphate.

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In solution, this physiologically useless reaction is 100 times as fast as isomerization. Hence, TIM must prevent the enediol from leaving the enzyme. This labile intermediate is trapped in the active site by the movement of a loop of 10 residues (see Figure 16.5). This loop serves as a lid on the active site, shutting it when the enediol is present and reopening it when isomerization is completed. We see here a striking example not only of catalytic perfection, but also of the acceleration of a desirable reaction so that it takes place much faster than an undesirable alternative reaction. Thus, two molecules of glyceraldehyde 3-phosphate are formed from one molecule of fructose 1,6-bisphosphate by the sequential action of aldolase and triose phosphate isomerase. The economy of metabolism is evident in this reaction sequence. The isomerase funnels dihydroxyacetone phosphate into the main glycolytic pathway—a separate set of reactions is not needed.

16.1.5. Energy Transformation: Phosphorylation Is Coupled to the Oxidation of Glyceraldehyde 3-phosphate by a Thioester Intermediate

The preceding steps in glycolysis have transformed one molecule of glucose into two molecules of glyceraldehyde 3-phosphate, but no energy has yet been extracted. On the contrary, thus far two molecules of ATP have been invested. We come now to a series of steps that harvest some of the energy contained in glyceraldehyde 3-phosphate. The initial reaction in this sequence is the conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate (1,3-BPG), a reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase (Figure 16.7).

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Figure 16.7. Structure of Glyceraldehyde 3-Phosphate Dehydrogenase.

Figure 16.7

Structure of Glyceraldehyde 3-Phosphate Dehydrogenase. Image mouse.jpg The active site includes a cysteine residue and a histidine residue adjacent to a bound NAD+.

Stage 3 of Glycolysis.

Figure

Stage 3 of Glycolysis. The oxidation of three-carbon fragments yields ATP.

1,3-Bisphosphoglycerate is an acyl phosphate. Such compounds have a high phosphoryl-transfer potential; one of its phosphoryl groups is transferred to ADP in the next step in glycolysis. The reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase is really the sum of two processes: the oxidation of the aldehyde to a carboxylic acid by NAD+ and the joining of the carboxylic acid and orthophosphate to form the acyl-phosphate product.

Image ch16fu12.jpg

The first reaction is quite thermodynamically favorable with a standard free-energy change, ΔG°´, of approximately -12 kcal mol-1 (-50 kJ mol-1), whereas the second reaction is quite unfavorable with a standard free-energy change of the same magnitude but the opposite sign. If these two reactions simply took place in succession, the second reaction would have a very large activation energy and thus not take place at a biologically significant rate. These two processes must be coupled so that the favorable aldehyde oxidation can be used to drive the formation of the acyl phosphate. How are these reactions coupled? The key is an intermediate, formed as a result of the aldehyde oxidation, that is higher in free energy than the free carboxylic acid is. This intermediate reacts with orthophosphate to form the acyl-phosphate product.

Let us consider the mechanism of glyceraldehyde 3-phosphate dehydrogenase in detail (Figure 16.8). In step 1, the aldehyde substrate reacts with the sulfhydryl group of cysteine 149 on the enzyme to form a hemithioacetal. Step 2 is the transfer of a hydride ion to a molecule of NAD+ that is tightly bound to the enzyme and is adjacent to the cysteine residue. This reaction is favored by the deprotonation of the hemithioacetal by histidine 176. The products of this reaction are the reduced coenzyme NADH and a thioester intermediate. This thioester intermediate has a free energy close to that of the reactants. In step 3, orthophosphate attacks the thioester to form 1,3-BPG and free the cysteine residue. This displacement occurs only after the NADH formed from the aldehyde oxidation has left the enzyme and been replaced by a second NAD+. The positive charge on the NAD+ may help polarize the thioester intermediate to facilitate the attack by orthophosphate.

Figure 16.8. Catalytic Mechanism of Glyceraldehyde 3-Phosphate Dehydrogenase.

Figure 16.8

Catalytic Mechanism of Glyceraldehyde 3-Phosphate Dehydrogenase. The reaction proceeds through a thioester intermediate, which allows the oxidation of glyceraldehyde to be coupled to the phosphorylation of 3-phosphoglycerate.

This example illustrates the essence of energy transformations and of metabolism itself: energy released by carbon oxidation is converted into high phosphoryl-transfer potential. The favorable oxidation and unfavorable phosphorylation reactions are coupled by the thioester intermediate, which preserves much of the free energy released in the oxidation reaction. We see here the use of a covalent enzyme-bound intermediate as a mechanism of energy coupling. A free-energy profile of the glyceraldehyde 3-phosphate dehydrogenase reaction, compared with a hypothetical process in which the reaction proceeds without this intermediate, reveals how this intermediate allows a favorable process to drive an unfavorable one (Figure 16.9).

Figure 16.9. Free-Energy Profiles for Glyceraldehyde Oxidation Followed by Acyl-Phosphate Formation.

Figure 16.9

Free-Energy Profiles for Glyceraldehyde Oxidation Followed by Acyl-Phosphate Formation. (A) A hypothetical case with no coupling between the two processes. The second step must have a large activation barrier, making the reaction very slow. (B) The actual (more...)

16.1.6. The Formation of ATP from 1,3-Bisphosphoglycerate

The final stage in glycolysis is the generation of ATP from the phosphorylated three-carbon metabolites of glucose. Phosphoglycerate kinase catalyzes the transfer of the phosphoryl group from the acyl phosphate of 1,3-bisphosphoglycerate to ADP. ATP and 3-phosphoglycerate are the products.

Image ch16fu13.jpg

The formation of ATP in this manner is referred to as substrate-level phosphorylation because the phosphate donor, 1,3-BPG, is a substrate with high phosphoryl-transfer potential. We will contrast this manner of ATP formation with that in which ATP is formed from ionic gradients in Chapters 18 and 19.

Thus, the outcomes of the reactions catalyzed by glyceraldehyde 3-phosphate dehydrogenase and phosphoglycerate kinase are:

1.

Glyceraldehyde 3-phosphate, an aldehyde, is oxidized to 3-phosphoglycerate, a carboxylic acid.

2.

NAD+ is concomitantly reduced to NADH.

3.

ATP is formed from Pi and ADP at the expense of carbon oxidation energy.

Keep in mind that, because of the actions of aldolase and triose phosphate isomerase, two molecules of glyceraldehyde 3-phosphate were formed and hence two molecules of ATP were generated. These ATP molecules make up for the two molecules of ATP consumed in the first stage of glycolysis.

16.1.7. The Generation of Additional ATP and the Formation of Pyruvate

In the remaining steps of glycolysis, 3-phosphoglycerate is converted into pyruvate with the concomitant conversion of ADP into ATP.

Image ch16fu14.jpg

The first reaction is a rearrangement. The position of the phosphoryl group shifts in the conversion of 3-phosphoglycerate into 2-phosphoglycerate, a reaction catalyzed by phosphoglycerate mutase. In general, a mutase is an enzyme that catalyzes the intramolecular shift of a chemical group, such as a phosphoryl group. The phosphoglycerate mutase reaction has an interesting mechanism: the phosphoryl group is not simply moved from one carbon to another. This enzyme requires catalytic amounts of 2,3-bisphosphoglycerate to maintain an active-site histidine residue in a phosphorylated form.

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The sum of these reactions yields the mutase reaction:

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Examination of the first partial reaction reveals that the mutase functions as a phosphatase—it converts 2,3-bisphosphoglycerate into 2-phosphoglycerate. However, the phosphoryl group remains linked to the enzyme. This phosphoryl group is then transferred to 3-phosphoglycerate to reform 2,3-bisphosphoglycerate.

In the next reaction, an enol is formed by the dehydration of 2-phosphoglycerate. Enolase catalyzes the formation of phosphoenolpyruvate (PEP). This dehydration markedly elevates the transfer potential of the phosphoryl group. An enol phosphate has a high phosphoryl-transfer potential, whereas the phosphate ester, such as 2-phosphoglycerate, of an ordinary alcohol has a low one. The ΔG°´ of the hydrolysis of a phosphate ester of an ordinary alcohol is -3 kcal mol-1 (- 13 kJ mol-1), whereas that of phosphoenolpyruvate is -14.8 kcal mol-1 (- 62 kJ mol-1). Why does phosphoenolpyruvate have such a high phosphoryl-transfer potential? The phosphoryl group traps the molecule in its unstable enol form. When the phosphoryl group has been donated to ATP, the enol undergoes a conversion into the more stable ketone—namely, pyruvate.

Image ch16fu15.jpg

Thus, the high phosphoryl-transfer potential of phosphoenolpyruvate arises primarily from the large driving force of the subsequent enol-ketone conversion. Hence, pyruvate is formed, and ATP is generated concomitantly. The virtually irreversible transfer of a phosphoryl group from phosphoenolpyruvate to ADP is catalyzed by pyruvate kinase. Because the molecules of ATP used in forming fructose 1,6-bisphosphate have already been regenerated, the two molecules of ATP generated from phosphoenolpyruvate are “profit.”

16.1.8. Energy Yield in the Conversion of Glucose into Pyruvate

The net reaction in the transformation of glucose into pyruvate is:

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Thus, two molecules of ATP are generated in the conversion of glucose into two molecules of pyruvate. The reactions of glycolysis are summarized in Table 16.3.

Table 16.3. Reactions of glycolysis.

Table 16.3

Reactions of glycolysis.

Note that the energy released in the anaerobic conversion of glucose into two molecules of pyruvate is -21 kcal mol-1 (- 88 kJ mol-1). We shall see in Chapters 17 and 18 how much more energy can be released from glucose in the presence of oxygen.

Conceptual Insights, Energetics of Glucose Metabolism

Image mouse.jpg See the section on the energetics of glycolysis in the conceptual insights module for a graphical representation of free energy differences among glycolytic metabolites, and how these differences are used to drive ATP and NADH synthesis in coupled reactions.

16.1.9. Maintaining Redox Balance: The Diverse Fates of Pyruvate

The conversion of glucose into two molecules of pyruvate has resulted in the net synthesis of ATP. However, an energy-converting pathway that stopped at pyruvate would not proceed for long, because redox balance has not been maintained. As we have seen, the activity of glyceraldehyde 3-phosphate dehydrogenase, in addition to generating a compound with high phosphoryl-transfer potential, of necessity leads to the reduction of NAD+ to NADH. There are limited amounts of NAD+ in the cell, which is derived from the vitamin niacin, a dietary requirement in human beings. Consequently, NAD+ must be regenerated for glycolysis to proceed. Thus, the final process in the pathway is the regeneration of NAD+ through the metabolism of pyruvate. The sequence of reactions from glucose to pyruvate is similar in most organisms and most types of cells. In contrast, the fate of pyruvate is variable. Three reactions of pyruvate are of prime importance: conversion into ethanol, lactic acid, or carbon dioxide (Figure 16.10).

Figure 16.10. Diverse Fates of Pyruvate.

Figure 16.10

Diverse Fates of Pyruvate. Ethanol and lactate can be formed by reactions involving NADH. Alternatively, a two-carbon unit from pyruvate can be coupled to coenzyme A (see Section 17.1.1) to form acetyl CoA.

Location of redox balance steps.

Figure

Location of redox balance steps. The generation and consumption of NADH, located within the glycolytic pathway.

1.

Ethanol is formed from pyruvate in yeast and several other microorganisms. The first step is the decarboxylation of pyruvate. This reaction is catalyzed by pyruvate decarboxylase, which requires the coenzyme thiamine pyrophosphate. This coenzyme, derived from the vitamin thiamine (B1), also participates in reactions catalyzed by other enzymes (Section 17.1.1). The second step is the reduction of acetaldehyde to ethanol by NADH, in a reaction catalyzed by alcohol dehydrogenase. This process regenerates NAD+.

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The active site of alcohol dehydrogenase contains a zinc ion that is coordinated to the sulfur atoms of two cysteine residues and a nitrogen atom of histidine (Figure 16.11). This zinc ion polarizes the carbonyl group of the substrate to favor the transfer of a hydride from NADH.

The conversion of glucose into ethanol is an example of alcoholic fermentation. The net result of this anaerobic process is:

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Note that NAD+ and NADH do not appear in this equation, even though they are crucial for the overall process. NADH generated by the oxidation of glyceraldehyde 3-phosphate is consumed in the reduction of acetaldehyde to ethanol. Thus, there is no net oxidation-reduction in the conversion of glucose into ethanol (Figure 16.12). The ethanol formed in alcoholic fermentation provides a key ingredient for brewing and winemaking.

2.

Lactate is formed from pyruvate in a variety of microorganisms in a process called lactic acid fermentation. The reaction also takes place in the cells of higher organisms when the amount of oxygen is limiting, as in muscle during intense activity. The reduction of pyruvate by NADH to form lactate is catalyzed by lactate dehydrogenase.

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The overall reaction in the conversion of glucose into lactate is:

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As in alcoholic fermentation, there is no net oxidation-reduction. The NADH formed in the oxidation of glyceraldehyde 3-phosphate is consumed in the reduction of pyruvate. The regeneration of NAD+in the reduction of pyruvate to lactate or ethanol sustains the continued operation of glycolysis under anaerobic conditions.

3.

Only a fraction of the energy of glucose is released in its anaerobic conversion into ethanol or lactate. Much more energy can be extracted aerobically by means of the citric acid cycle and the electron-transport chain. The entry point to this oxidative pathway is acetyl coenzyme A (acetyl CoA), which is formed inside mitochondria by the oxidative decarboxylation of pyruvate.

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This reaction, which is catalyzed by the pyruvate dehydrogenase complex, will be discussed in detail in Chapter 18. The NAD+ required for this reaction and for the oxidation of glyceraldehyde 3-phosphate is regenerated when NADH ultimately transfers its electrons to O2 through the electron-transport chain in mitochondria.

Figure 16.11. Active Site of Alcohol Dehydrogenase.

Figure 16.11

Active Site of Alcohol Dehydrogenase. The active site contains a zinc ion bound to two cysteine residues and one histidine residue. The zinc ion binds the acetaldehyde substrate through its oxygen atom, polarizing it so that it more easily accepts a hydride (more...)

Figure 16.12. Maintaining Redox Balance.

Figure 16.12

Maintaining Redox Balance. The NADH produced by the glyceraldehyde 3-phosphate dehydrogenase reaction must be reoxidized to NAD+ for the glycolytic pathway to continue. In alcoholic fermentation, alcohol dehydrogenase oxidizes NADH and generates ethanol. (more...)

16.1.10. The Binding Site for NAD+ Is Similar in Many Dehydrogenases

Image tree.jpg Although the enzymes taking part in glycolysis and the subsequent conversion of pyruvate are structurally diverse, the three dehydrogenases—glyceraldehyde 3-phosphate dehydrogenase, alcohol dehydrogenase, and lactate dehydrogenase—have in common a domain for NAD+ binding (Figure 16.13). This nucleotide-binding region is made up of four α helices and a sheet of six parallel β strands. Moreover, in all cases, the bound NAD+ displays nearly the same conformation. This common structural domain, one of the first recurring structural domains to be discovered, is often called a Rossmann fold after Michael Rossmann, who first recognized it. This fold likely represents a primordial dinucleotide-binding domain that recurs in the dehydrogenases of glycolysis and other enzymes because of their descent from a common ancestor.

Figure 16.13. NAD+ -binding region in dehydrogenases.

Figure 16.13

NAD+ -binding region in dehydrogenases. Image mouse.jpg The nicotinamide-binding half (yellow) is structurally similar to the adenine-binding half (red). The two halves together form a structural motif called a Rossmann fold. The NAD+ molecule binds in an extended conformation. (more...)

16.1.11. The Entry of Fructose and Galactose into Glycolysis

Although glucose is the most widely used monosaccharide, others also are important fuels. Let us consider how two abundant sugars—fructose and galactose—can be funneled into the glycolytic pathway (Figure 16.14). Much of the ingested fructose is metabolized by the liver, using the fructose 1-phosphate pathway (Figure 16.15). The first step is the phosphorylation of fructose to fructose 1-phosphate by fructokinase. Fructose 1-phosphate is then split into glyceraldehyde and dihydroxyacetone phosphate, an intermediate in glycolysis. This aldol cleavage is catalyzed by a specific fructose 1-phosphate aldolase. Glyceraldehyde is then phosphorylated to glyceralde-hyde 3-phosphate, a glycolytic intermediate, by triose kinase. Alternatively, fructose can be phosphorylated to fructose 6-phosphate by hexokinase. However, the affinity of hexokinase for glucose is 20 times as great as it is for fructose. Little fructose 6-phosphate is formed in the liver because glucose is so much more abundant in this organ. Moreover, glucose, as the preferred fuel, is also trapped in the muscle by the hexokinase reaction. Because liver and muscle phosphorylate glucose rather than fructose, adipose tissue is exposed to more fructose than glucose. Hence, the formation of fructose 6-phosphate is not competitively inhibited to a biologically significant extent, and most of the fructose in adipose tissue is metabolized through fructose 6-phosphate.

Figure 16.14. Entry Points in Glycolysis for Galactose and Fructose.

Figure 16.14

Entry Points in Glycolysis for Galactose and Fructose.

Figure 16.15. Fructose Metabolism.

Figure 16.15

Fructose Metabolism. Fructose enters the glycolytic pathway in the liver through the fructose 1-phosphate pathway.

There are no catabolic pathways to metabolize galactose, so the strategy is to convert galactose into a metabolite of glucose. Galactose is converted into glucose 6-phosphate in four steps. The first reaction in the galactose-glucose interconversion pathway is the phosphorylation of galactose to galactose 1-phosphate by galactokinase.

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Galactose 1-phosphate then acquires a uridyl group from uridine diphosphate glucose (UDP-glucose), an intermediate in the synthesis of glycosidic linkages (Section 21.4.2).

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The products of this reaction, which is catalyzed by galactose 1-phosphate uridyl transferase, are UDP-galactose and glucose 1-phosphate. The galactose moiety of UDP-galactose is then epimerized to glucose. The configuration of the hydroxyl group at carbon 4 is inverted by UDP-galactose 4-epimerase.

The sum of the reactions catalyzed by galactokinase, the transferase, and the epimerase is:

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Note that UDP-glucose is not consumed in the conversion of galactose into glucose, because it is regenerated from UDP-galactose by the epimerase. This reaction is reversible, and the product of the reverse direction also is important. The conversion of UDP-glucose into UDP-galactose is essential for the synthesis of galactosyl residues in complex polysaccharides and glycoproteins if the amount of galactose in the diet is inadequate to meet these needs.

Finally, glucose 1-phosphate, formed from galactose, is isomerized to glucose 6-phosphate by phosphoglucomutase. We shall return to this reaction when we consider the synthesis and degradation of glycogen, which proceeds through glucose 1-phosphate, in Chapter 21.

16.1.12. Many Adults Are Intolerant of Milk Because They Are Deficient in Lactase

Image caduceus.jpg Many adults are unable to metabolize the milk sugar lactose and experience gastrointestinal disturbances if they drink milk. Lactose intolerance, or hypolactasia, is most commonly caused by a deficiency of the enzyme lactase, which cleaves lactose into glucose and galactose.

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“Deficiency” is not quite the appropriate term, because a decrease in lactase is normal during development in all mammals. As children are weaned and milk becomes less prominent in their diets, lactase activity normally declines to about 5 to 10% of the level at birth. This decrease is not as pronounced with some groups of people, most notably Northern Europeans, and people from these groups can continue to ingest milk without gastrointestinal difficulties. With the appearance of milk-producing domesticated animals, a human being with a genetic alteration endowing high levels of lactase activity in adulthood would hypothetically have a selective advantage in being able to consume calories from the readily available milk.

Scanning electron micrograph of Lactobacillus.

Figure

Scanning electron micrograph of Lactobacillus. The anaerobic bacteria Lactobacillus is shown here (artificially colored) at a magnification of 22, 245×. As suggested by its name, this genus of bacteria ferments glucose into lactic acid, and is (more...)

What happens to the lactose in the intestine of a lactase-deficient person? The lactose is a good energy source for microorganisms in the colon, and they ferment it to lactic acid while also generating methane (CH4) and hydrogen gas (H2). The gas produced creates the uncomfortable feeling of gut distention and the annoying problem of flatulence. The lactic acid produced by the microorganisms is osmotically active and draws water into the intestine, as does any undigested lactose, resulting in diarrhea. If severe enough, the gas and diarrhea hinder the absorption of other nutrients such as fats and proteins. The simplest treatment is to avoid the consumption of products containing much lactose. Alternatively, the enzyme lactase can be ingested with milk products.

16.1.13. Galactose Is Highly Toxic If the Transferase Is Missing

Image caduceus.jpg Less common than lactose intolerance are disorders that interfere with the metabolism of galactose. The disruption of galactose metabolism is referred to as galactosemia. The most common form, called classic galactosemia, is an inherited deficiency in galactose 1-phosphate uridyl transferase activity. Afflicted infants fail to thrive. They vomit or have diarrhea after consuming milk, and enlargement of the liver and jaundice are common, sometimes progressing to cirrhosis. Cataracts will form, and lethargy and retarded mental development also are common. The blood-galactose level is markedly elevated, and galactose is found in the urine. The absence of the transferase in red blood cells is a definitive diagnostic criterion.

The most common treatment is to remove galactose (and lactose) from the diet. The enigma of galactosemia is that, although elimination of galactose from the diet prevents liver disease and cataract development, the majority of patients still suffer from central nervous system malfunction, most commonly a delayed acquisition of language skills. Females will also display ovarian failure.

Cataract formation is better understood. A cataract is the clouding of the normally clear lens of the eye. If the transferase is not active in the lens of the eye, the presence of aldose reductase causes the accumulating galactose to be reduced to galactitol.

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Galactitol is osmotically active, and water will diffuse into the lens, instigating the formation of cataracts. In fact, there is a high incidence of cataract formation with age in populations that consume substantial amounts of milk into adulthood.

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