Aspartate aminotransferase active site structure

Aspartate aminotransferase 57s, 135s, 753 absorption spectra 749 active site structure 744 atomic structure 750 catalytic intermediates, models 752 NMR spectra 149 quinonoid intermediate 750 Ramachandran plot 61 sequence 57 transamination 742 Aspartate ammonia-lyase 685 Aspartate carbamoyltransferase 348s active sites 348 regulation 540... 

Active site structure of Escherichia coli aspartate aminotransferase (see Chapter 5 for details). 

Fig. 7. (Top) View of the active-site structure of chicken heart mitochondrial aspartate aminotransferase with bound L-aspartate. (Bottom) Computer graphics model of the hypothetical active-site structure of E. coli Arg-292 - Asp (R292D) mutant aspartate aminotransferase with L-arginine bound. [Reprinted with permission from Ref. (60).]...

Figure 14.33 Active site structures of (a) aspartate aminotransferase [92] and (b) alanine racemase [93], Italic distance values were derived by X-ray crystallography and bold values...

Figure 14-6 Drawing showing pyridoxal phosphate (shaded) and some surrounding protein structure in the active site of cytosolic aspartate aminotransferase. This is the low pH form of the enzyme with an N-protonated Schiff base linkage of lysine 258 to the PLP. The tryptophan 140 ring lies in front of the coenzyme. Several protons, labeled Ha, Hb, and Hd are represented in NMR spectra by distinct resonances whose chemical shifts are sensitive to changes in the active site.169...

Many of the enzymes that catalyze these reactions, such as serine hy- droxymethyltransferase, which converts serine into glycine, have the same fold as that of aspartate aminotransferase and are clearly related by divergent evolution. Others, such as tryptophan synthetase, have quite different overall structures. Nonetheless, the active sites of these enzymes are remarkably similar to that of aspartate aminotransferase, revealing the effects of convergent evolution. 

The a subunit catalyzes the formation of indole from indole-3-glycerol phosphate, whereas each P subunit has a PLP-containing active site that catalyzes the condensation of indole and serine to form tryptophan. The overall three-dimensional structure of this enzyme is distinct from that of aspartate aminotransferase and the other PLP enzymes already discussed. Serine forms a Schiff base with this PLP, which is then dehydrated to give the Schiffbase of aminoacrylate. This reactive intermediate is attacked by indole to give tryptophan. 

Aspartate aminotransferase (AAT) is the first PLP-dependent enzyme for which the three-dimensional structure has been determined " " and is the prototype of fold-type I PLP-enzymes. Each subunit of the AAT homodimer has a large and a small domain. The coenzyme is bound to the large (N-terminal) domain and located in a pocket at the subunit interface, so that residues from each monomer contribute to the formation of both active sites. The proximal and distal carboxylate group of the dicarboxylic substrates bind to Arg386 and Arg292, respectively, the latter contributed by the opposite subunit. " Early crystallographic strucmres... 

The crystal structure of kynureninase from P. fluorescens was solved in 2004. The enzyme shares the same structural fold as aspartate aminotransferase, but shares low sequence similarity. An active site arginine residue (Arg-375) was identified, which is important in substrate binding. The structure of the human kynureninase, which shows a catalytic preference for 3-hydroxy-kynurenine over L-kynurenine, was solved in 2007. The human enzyme shares the same fold as the P. fluorescens enzyme, and also contains an active site arginine residue (Arg-434). The catalytic mechanism requires two acid/base residues, which have not yet been unambiguously assigned. The hydrolytic cleavage step is believed to proceed via a general base mechanism. ... 

The most recent crystallographic study discloses the structure of the methylamine oxidase from the yeast Hansenula polymorpha [31], which shows an integrated network of water molecules providing electron transfer from topa quinone to copper and other important features such as the channel for oxygen entry and hydrogen peroxide release. The role of the active site aspartate base (Asp319) in the aminotransferase mechanism of the copper amine oxidase from H. polymorpha has been probed by site-directed mutagenesis [141]. It has been demonstrated by several... 

D. L. Smith, S. C. Almo, M. D. Toney and D. Ringe, 2.8-A-resolution crystal structure of an active-site mutant of aspartate aminotransferase from Escherichia coli, Biochemistry, 1989, 28, 8161-8167 (pdb 2aat). 

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