The number of known three-dimensional structures of vitamin B-6-dependent e
nzymes has doubted in the past two years. A fourth type of fold for BG-depe
ndent enzymes, involving a TIM-barrel domain, has been discovered. Alanine
racemase is the first known representative of this new fold. Significant pr
ogress has been made in understanding the allosteric effects in the tryptop
han synthase reaction.