Pyridoxal-5'-phosphate-dependent enzymes (B6 enzymes) catalyze manifold reactions in the metabolism of amino acids. Most of these enzymes can be assigned to one of three different families of homologous proteins, the alpha, beta and gamma families. The alpha and gamma family might be distantly related with one another, but are clearly not homologous with the beta family. The beta family includes L- and D-serine dehydratase, threonine dehydratase, the beta subunit of tryptophan synthase, threonine synthase and cysteine synthase. These enzymes catalyze beta-replacement or beta-elimination reactions [ 8112347].
Comparison of sequences from eukaryotic, archebacterial, and eubacterial species indicates that the functional specialization of most B6 enzymes has occurred already in the universal ancestor cell. The cofactor pyridoxal-5-phosphate must have emerged very early in biological evolution; conceivably, organic cofactors and metal ions were the first biological catalysts [10800595].
The 3D structure of the beta-subunit of tryptophan synthase has been solved. The subunit has two domains that are approximately the same size and similar to
each other in folding pattern. Each has a core containing a four-stranded
parallel beta-sheet with three helices on its inner side and one on the outer
side. The cofactor is bound at the interface between the domains [7748903]. | 
0
