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On the basis of the recently determined tertiary structures of the active site of three transaminases, aspartate transaminase (AST), branched-chain amino acid transaminase (BCT)) from E. coli and aro matic amino acid transaminase (ART) from Paracoccus denitrificans, I describe the mode of substrate recognition and propose a new concept that may be listed as one of the factors for enhancing the catalytic ability of the enzyme. The α-and ω-carboxylate groups of acidic-amino acid substrates bind to R386 and R292, respec tively, in AST and ART, by forming salt bridge of "end-on" symmetric geometry. In ART, aromatic side chains of aromatic substrates occupy the same site as the ω-carboxylate group of the acidic substrate, causing reorientation of the side chains of a few active site residues and of the position of water molecules to form a new hydrogen bonding network in contrast to the active site structure of the enzyme-acidic substrate complex. In BCT, the α-carboxylate group of the substrate forms hydrogen bonds with two main chain NH groups of T257 and A258 with the aid by R40. In the active site of this enzyme, there is a pocket of which structure allows to accommodate hydrophobic side chain of the branched-chain substrates and the to-carboxylic group of glutamate. The X-ray structure of the active site of AST shows the strained conformation of the protonated internal aldimine provided by K258. We found that this strain lowers the pK_a of the internal aldimine of AST and that the pK_a increases progressively from the unliganded to the external aldimine com plex. We propose that this strain enhances the catalytic ability of AST by increasing the energy level of the free enzyme plus substrate at pH7 relative to the transition state.

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