Type III Pyridoxal 5-phosphate (PLP)-Dependent Enzymes
The fold type III PLP-dependent enzyme family is predominantly composed of two-domain proteins with similarity to bacterial alanine racemases (AR) including eukaryotic ornithine decarboxylases (ODC), prokaryotic diaminopimelate decarboxylases (DapDC), biosynthetic arginine decarboxylases (ADC), carboxynorspermidine decarboxylases (CANSDC), and similar proteins. AR-like proteins contain an N-terminal PLP-binding TIM-barrel domain and a C-terminal beta-sandwich domain. They exist 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. These proteins play important roles in the biosynthesis of amino acids and polyamine. The family also includes the single-domain YBL036c-like proteins, which contain a single PLP-binding TIM-barrel domain without any N- or C-terminal extensions. Due to the lack of a second domain, these proteins may possess only limited D- to L-alanine racemase activity or non-specific racemase activity.