Type III Pyridoxal 5-phosphate (PLP)-Dependent Enzyme D-Threonine Aldolase
D-threonine aldolase (D-TA, EC 188.8.131.52) reversibly catalyzes the aldol cleavage of D-threonine into glycine and acetaldehyde, and the synthesis of D-threonine from glycine and acetaldehyde. Its activity is present in several genera of bacteria but not in fungi. It requires PLP and a divalent cation such as Co2+, Ni2+, Mn2+, or Mg2+ as cofactors for catalytic activity and thermal stability. Members of this subfamily show similarity to bacterial alanine racemase (AR), a fold type III PLP-dependent enzyme which contains an N-terminal PLP-binding TIM-barrel domain and a C-terminal beta-sandwich domain. AR exists as homodimers with active sites that lie at the interface between the TIM barrel domain of one subunit and the beta-sandwich domain of the other subunit. Based on its similarity to AR, it is possible that low specificity D-TAs also form dimers in solution. Experimental data show that the monomeric form of low specificity D-TAs exhibit full catalytic activity.