1akc
STRUCTURAL BASIS FOR THE CATALYTIC ACTIVITY OF ASPARTATE AMINOTRANSFERASE K258H LACKING ITS PYRIDOXAL-5'-PHOSPHATE-BINDING LYSINE RESIDUE
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OverviewOverview
Chicken mitochondrial and Escherichia coli aspartate aminotransferases, K258H, in which the active site lysine residue has been exchanged for a, histidine residue, retain partial catalytic competence [Ziak et al. (1993), Eur. J. Biochem. 211, 475-484]. Mutant PLP and PMP holoenzymes and the, complexes of the latter (E. coli enzyme) with sulfate and 2-oxoglutarate, as well as complexes of the mitochondrial apoenzyme with, N-(5'-phosphopyridoxyl)-L-aspartate or, N-(5'-phosphopyridoxyl)-L-glutamate, were crystallized and analyzed by, means of X-ray crystallography in order to examine how the side chain of, histidine 258 can substitute as a general acid/base catalyst of the, aldimine-ketimine tautomerization in enzymic transamination. The, structures have been solved and refined at resolutions between 2.1 and 2.8, A. Both the closed and the open conformations, identical to those of the, wild-type enzyme, were observed, indicating that the mutant enzymes of, both species exhibit the same conformational flexibility as the wild-type, enzymes, although in AspAT K258H the equilibrium is somewhat shifted, toward the open conformation. The replacement of the active site K258 by a, histidine residue resulted only in local structural adaptations necessary, to accommodate the imidazole ring. The catalytic competence of the mutant, enzyme, which in the forward half-reaction is 0.1% of that of the, wild-type enzyme, suggests that the imidazole group is involved in the, aldimine-ketimine tautomerization. However, the imidazole ring of H258 is, too far away from C alpha and C4' of the coenzyme-substrate adduct for, direct proton transfer, suggesting that the 1,3-prototropic shift is, mediated by a water molecule. Although there is enough space for a water, molecule in this area, it has not been detected. Dynamic fluctuations of, the protein matrix might transiently open a channel, giving a water, molecule fleeting access to the active site.
About this StructureAbout this Structure
1AKC is a Single protein structure of sequence from Gallus gallus with PPE as ligand. Active as Aspartate transaminase, with EC number 2.6.1.1 Full crystallographic information is available from OCA.
ReferenceReference
Structural basis for the catalytic activity of aspartate aminotransferase K258H lacking the pyridoxal 5'-phosphate-binding lysine residue., Malashkevich VN, Jager J, Ziak M, Sauder U, Gehring H, Christen P, Jansonius JN, Biochemistry. 1995 Jan 17;34(2):405-14. PMID:7819232
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