Adenosyltransferase,

SuFRiN, J.R., Dunn, D.A., Marshall, G.R. Steric mapping of the L-methionine binding site of ATP L-methionine S-adenosyltransferase. Mol. Pharmacol. 1981,19, 307-313. 

A sulfonium derivative, abbreviated AdoMet or SAM, that is primarily generated by the action of methionine adenosyltransferase (ATP + L-methionine + H2O SAM + Pi + pyrophosphate). SAM is a major methylat-... 

This enzyme [EC 2.5.1.17], also referred to as cob(I)-alamin adenosyltransferase or aquacob(I)alamin adeno-syltransferase, catalyzes the reaction of cob(I)alamin with ATP and water to produce adenosylcobalamin, orthophosphate, and pyrophosphate (or, diphosphate). A cofactor for this enzyme is manganese ion. 

CARNOSINE SYNTHETASE CHAPERONES CHOLINE KINASE CHOLOYL-CoA SYNTHETASE COBALAMIN ADENOSYLTRANSFERASE 4-COUMAROYL-CoA SYNTHETASE CREATINE KINASE CTP SYNTHETASE CYTIDYLATE KINASE 2-DEHYDRO-3-DEOXYGLUCONOKINASE DEHYDROGLUCONOKINASE DEOXYADENOSINE KINASE DEOXYADENYLATE KINASE DEOXYCYTIDINE KINASE (DEOXYjNUCLEOSIDE MONOPHOSPHATE KINASE DEOXYTHYMIDINE KINASE DEPHOSPHO-CoA KINASE DETHIOBIOTIN SYNTHASE DIACYLGLYCEROL KINASE DIHYDROFOLATE SYNTHETASE DNA GYRASES DNA REVERSE GYRASE ETHANOLAMINE KINASE EXONUCLEASE V... 

METHIONINE ADENOSYLTRANSFERASE METHIONINE y-LYASE METHIONINE SULFOXIDE REDUCTASE Methionine synthase,... 

CELLOBIOSE PHOSPHORYLASE CHORISMATE SYNTHASE COBALAMIN ADENOSYLTRANSFERASE 3-DEHYDROQUINATE SYNTHASE... 

CHOLOYL-CoA SYNTHETASE COBALAMIN ADENOSYLTRANSFERASE 4-COUMAROYL-CoA SYNTHETASE... 

METHIONINE ADENOSYLTRANSFERASE METHIONYL-tRNA SYNTHETASE NAD SYNTHETASE... 

SAM) 591,592s, 813, 875 decarboxylation 753, 754 reaction with catechol O-methyltransferase 592 Adenosyltransferase, B12s 870 Adenoviruses 247, 346 Adenylate cyclase 556-557, 556s, 657 characterization of 556 toxic 548... 

Fig. 1. Ethylene biosynthesis. The numbered enzymes are (1) methionine adenosyltransferase, (2) ACC (l-aminocyclopropane-l-carboxylic acid) synthase, (3) ethylene forming enzyme (EFE), (4) 5 -methylthio-adenosine nucleosidase, (5) 5 -methylthioribose kinase. Regulation of the synthesis of ACC synthase and EFE are important steps in the control of ethylene production. ACC synthase requires pyridoxal phosphate and is inhibited by aminoethoxy vinyl glycine EFE requires 02 and is inhibited under anaerobic conditions. Synthesis of both ACC synthase and EFE is stimulated during ripening, senescence, abscission, following mechanical wounding, and treatment with auxins.

Glutamine transaminase from bovine liver, one of the enzymes involved in methionine catabolism, utilizes SeMet as well as methionine (Blazon et al., 1994). However, with some enzymes, differences in the reaction rates for SeMet and Met have been observed. For example, SeMet is a better substrate than Met for the a,7-elimination by L-methionine 7-lyase of Pseudomonas putida (Esaki et al., 1979). The adenosyltransferase from rat liver reacts with L(+)-SeMet at 51% of the rate with L(+)-Met, and with the corresponding d(—) isomers at only 13 and 10% of the rate of L-Met (Pan and Tarver, 1967). The adenosyl transferase from yeast, on the other hand, is more active with SeMet than with Met (Mudd and Cantoni, 1957). This enzyme produces the... 

Pan, F. and Tarver, H. 1967. Comparative studies on methionine, selenomethionine and the ethyl analogues as substrates for methionine adenosyltransferase from rat liver. Arch. Biochem. Biophys. 119, 429-434. 

SAM) by methionine adenosyltransferase. SAM serves as a methyl donor for a variety of methyl acceptors, including DNA, protein, neurotransmit-ters, and phospholipids. 5-Adenosylhomocysteine (SAH) is produced following methyl donation by SAM, and homocysteine is formed through the liberation of adenosine from SAH by the enzyme SAH hydrolase. Unlike methionine and cysteine, homocysteine is not incorporated into polypeptide chains during protein synthesis. Instead, homocysteine has one of two metabolic fates transsulfuration or remethylation to methionine. 

Figure 21-1. Structural and metabolic relationships between methionine, homocysteine, and cysteine. CBS, cystathionine b-synthase CTH, cystathionine y-lyase MAT, methionine adenosyltransferase MS, methionine synthase 5-MTHF, 5-methyltetrahydrofoIate MTs, methyl transferases PLR pyridoxal phosphate SAH, S-adenosylhomocysteine SAHH, SAH hydrolase THF, tetrahydrofolate.

Figure 10.9. Metabolism of methionine. Methionine adenosyltransferase, EC 2.5.1.6 methionine synthetase, EC 2.1.1.13 (vitamin B12-dependent) and EC 2.1.1.5 (betaine as a methyl donor) cystathionine, 6-synthetase, EC 4.2.1.22 and y-cystathionase, EC 4.4.I.I.

Protection required. Suppose that a mutation in bacteria resulted in diminished activity of methionine adenosyltransferase, the enzyme responsible for the synthesis of SAM from methionine and ATP. Predict how this might affect the stability of the mutated bacteria s DNA. 

A Bj2s adenosyltransferase catalyzes nucleophilic displacement on the 5 carbon of ATP with formahon of fhe coenzyme and displacement of inorganic fri-polyphosphate PPP . 

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