Aspartate aminotransferase catalytic

Catalytic domain 1 doubly wound parallel fi sheet Catalytic domain 2 classic doubly wound fi sheet (Fig. 76) Aspartate transaminase see Aspartate aminotransferase Aspartate transcarbamylase (Monaco et ah, 1978), see Aspartate carbamoyltransferase... 

Figure 14-10 Models of catalytic intermediates for aspartate aminotransferase in a half-transamination reaction from aspartate to oxalocetate. For clarity, only a selection of the active site groups are shown. (A) Michaelis complex of PLP enzyme with aspartate. (B) Geminal diamine. (C) Ketimine intermediate. The circle indicates a bound water molecule. See Jansonius and Vincent in Jurnak and McPherson.163 Courtesy of J.N. Jansonius.

Below the active site of aspartate aminotransferase, as shown in Fig. 14-6, is a cluster of three buried histidine side chains in close contact with each other. The imidazole of H143 is hydrogen bonded to the D222 carboxylate, the same carboxylate that forms an ion pair with the coenzyme. This system looks somewhat like the catalytic triad of the serine proteases in reverse. As with the serine proteases, the proton-labeled Hb in Fig. 14-6 can be "seen" by NMR spectroscopy (Fig. 3-30). So can the proton Ha on the PLP ring. These protons... 

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... 

Thiosulfate cyanide sulfurtransferase symmetry in 78 TTiiouridine 234 Three-dimensional structures of aconitase 689 adenylate kinase 655 aldehyde oxido-reductase 891 D-amino acid oxidase 791 a-amylase, pancreatic 607 aspartate aminotransferase 57,135 catalytic intermediates 752 aspartate carbamyltransferase 348 aspartate chemoreceptor 562 bacteriophage P22 66 cadherin 408 calmodulin 317 carbonic acid anhydrase I 679 carboxypeptidase A 64 catalase 853 cholera toxin 333, 546 chymotrypsin 611 citrate synthase 702, 703 cutinase 134 cyclosporin 488 cytochrome c 847 cytochrome c peroxidase 849 dihydrofolate reductase 807 DNA 214, 223,228,229, 241 DNA complex... 

The interconversion of o -ketoglutarate to glutamate involves the malate-aspartate shutde. This shuttle translocates a-ketoglutarate from mitochondria into the cytoplasm and then converts it to glutamate by the catalytic action of aspartate aminotransferase (McKenna et al., 2006). As part of the malate-aspartate shuttle, NADH is oxidized during reduction of oxaloacetate to malate. Malate diffuses across the outer mitochondrial membrane (Fig. 1.6). From the intermembrane space, the malate-a-ketoglutarate antiporter in the inner membrane transports malate into the matrix. For every malate molecule entering the matrix compartment, one molecule of... 

Branched-chain aminotransferases (BCATs) evolved from aspartate aminotransferases (AATs) showed a record 105- to 2 x 106-fold improvement in catalytic efficiency (kcat/KM). Not only were the 13-17 amino acid substitutions concentrated in the most active mutants, but all but one mutated amino acid residues are located far from the active site. With directed evolution, enantioselectivities can be improved on enantiounspecific enzymes (from E = 1.1 to 25.8) and even inverted to yield the opposite enantiomer in comparison to the wild type (40% d- to both 90% d- and 20% L-). 

Fasella P and Turano C (1970) Structure and catalytic role of the functional groups of aspartate aminotransferase. Vitamins and Hormones 157-94. 

Schumann G, Bonora R, Ceriotti F, F6rard G, Ferrero CA, Franck PFH, et al. IFCC primary reference procedures for the measurement of catalytic activity concentrations of enzymes at 37°C. Part 5. Reference procedure for the measurement of catalytic concentration of aspartate aminotransferase. Clin Chem Lab Med 2002 40 725-33. 

Immunological methods for enzymes, more specifically isoenzymes, such as lactate dehydrogenase-1 (167, 168), mitochondrial aspartate aminotransferase (169), prostatic acid phosphatase (170, 171,172), and creatine kinase-MB (173, 174, 175), have been in use in the clinical laboratory for 10 years. However, the use of the immunological rather than catalytic properties of enzymes has not provided the opportunities for standardization that was anticipated a number of years ago (176, 177, 178). It is only within the last year that a working group on CK-MB mass assay was formed under the auspices of the Standards Committee of the American Association for Clinical Chemistry (AACC). The objective of this working group is to prepare a reference material to calibrate methods that are based on the principle of CK-MB mass measurement. 

Although no X-ray crystal structure is available for a PLP-dependent decarboxylase, the X-ray structure of mitochondrial aspartate aminotransferase (775, 116) can be used to propose a likely picture of the catalytic mechanism for the analogous glutamate decarboxylase (Scheme IX). 

Fig. 11. Proposed catalytic mechanism for aspartate aminotransferase deduced on the basis of the crystallographic structures of the holoenzyme (mitochondria) and its complexes with substrate analogs (242). Model building was used to formulate the most probable structures formed along the reaction pathway with normal substrates.

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 experiments conclusively prove that the addition of hydrogen to C-4 of the coenzyme occurs at the Si face of the Schiff base. Evidence has already been provided for the syn nature of the tautomeric process in the reaction catalysed by pyridoxamine-pyruvate aminotransferase [107]. If the same precedent is extended to aspartate aminotransferase it then follows that the bond to C that is formed and broken in this case must also be located on the Si face at C-4, in the catalytic complex, as shown in structure 2 (Fig. 53). In other words, the alternative arrangement for syn proton transfer shown in 1 (Fig. 53) is precluded by these experiments. The direction of hydrogen addition to C-4 of the coenzyme in the half-reaction has also been studied using several other L-amino add requiring aminotransferases and in every case the medium hydrogen was shown to add to the Si face at C-4 (Table 5). These experiments have led to the generaUsed view that in B -dependent reactions... 

Figure 11.13 Reactions at a-carbon of a-amino acids catalyzed by pyridoxal enzymes All three substituents at C are subject to labilization in the three types of a-carbon reactions. The hydrogen is labilized in recemization reactions, the amino group is labUized in the transamination and the carboxyl group is labilized in decarboxylation. a-Amino acid condenses with pyridoxal phosphate to yield pyridoxylidene imino acid (an aldimine). The common intermediate, aldimine and distinct ketimines leading to the production of oxo-acid (in transamination), amino acid (in racemization) and amine (in decarboxylation) are shown. The catalytic acid (H-A-) and base (-B ) are symbolic both can be from the same residue such as Lys258 in aspartate aminotransferase.

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