Precise structure and double substrate recognition of pyridoxal protein

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X-ray crystallographic studies of PLP-dependent aminotransferases have been performed in order to elucidate the mechanism for double substrate recognition. The enzymes seem to have various and complex methods to recognize two kinds of amino acids, the side chains of which are different in size and property, from many other small molecules. However, there are two fundamental mechanisms. The fundamental mechanisms have been observed in aromatic and branched-chain amino acid aminotransferases (AroAT and BCAT, respectively). The former shows the large-scale rearrangement of hydrogen-bonding network at the active site (switch mechanism), while the latter recognizes two kinds of substrate without changing the folding of the main-chain and side-chain conformation (as-is mechanism). Either of AroAT or BCAT binds hydrophobic and acidic side chains of the substrates at the same place of the active site. In AroAT, the guanidino group of Arg makes a salt bridge with the carboxylate of the substrat … More e (glutamate). When the hydrophobic side chain is bound to the same place as that of the carboxylate, the guanidino group moves away with the concomitant reconstruction of the active-site pocket. In BCAT, the hydrophilic side chain is hydrogen bonded to the hydrophilic sites of the pocket which are in the vicinity of the solvent side. The pocket is hydrophobic as a whole and may encapsulate the hydrophobic side chain inside the pocket without reconstruction of the active site pocket.The double substrate recognition mechanism in histidinol phosphate aminotransferase is essentially the same as that of BCAT, although small-scale induced-fit is associated with the recognition. The active site is designed as the lock for both acidic and basic side chains as keys. As is observed in the case for BCAT, the side-chain carboxylate of Glu binds to the place adjacent to the solvent region. The recognition method in glutamine aminotransferase is the same as that in BCAT. The hydrophobic pocket, the inside of which has hydrophiilic sites, can accommodate hydrophobic and hydrophilic side chains. Acetylornithine aminotransferase is unusual in that the enzyme lacks ct-carboxylate. In this case, a hybrid of as-is and switch mechanism is used to recognize acetylornithine and glutamate. Less