Adenosylmethionine formation

Parry RJ, Minta A. Studies of enzyme stereochemistry-elucidation of the stereochemistry of S-adenosylmethionine formation by yeast methionine adenosyltransferase. J. Am. Chem. Soc. 1982 104 871-872. 

Figure 30-17. Formation of S-adenosylmethionine. -CHj represents the high group transfer potential of "active methionine."...

Despite our earlier failure in formate feeding experiments, [3- C]serine, [1,2- CJglycine, and [Me- C]methionine were found to enrich C-13 in neosaxitoxin effectively (7). The best incorporation was observed with methionine, indicating it is the direct precursor via S-adenosylmethionine. Glycine C-2 and serine C-3 must have been incorporated through tetrahydrofolate system as methyl donors in methionine biosynthesis. 

The compounds that are the immediate methyl gronp donors are methyltetra hydrofate (CH3-FH4) and S-adenosyl methionine (SAM) (see Figure 15.2). These are involved in, at least, five key reactions or processes which are summarised in Figure 15.4. Complexity arises in the topic of methyl group transfer in formation and reformation of the methylating compounds 5-adenosylmethione and methyl tetrahydrofolate. There are four important reactions in the formation utilisation and then the reformation of 5-adenosylmethionine as follows ... 

In biological methylation, the 5-methyl group of the amino acid L-methionine is used to methylate suitable O, N, S, and C nucleophiles. First, methionine is converted into the methylating agent S-adenosylmethionine (SAM). SAM is nucleoside derivative (see Section 14.3). Both the formation of SAM and the subsequent methylation reactions are nice examples of biological Sn2 reactions. 

The enzymes that utilise Fe-S clusters and S-adenosylmethionine to generate radicals essential for catalysis are now identified as a class or superfamily, the radical-SAM enzymes. Hoffman et al. have studied the pyruvate formate-lyase activating enzyme (PFL-AE) by cw EPR (X-band) and pulsed ENDOR (2H and 13C, Q-band) and used the S = signals of the Fe4S4 cluster and derivatives to construct a model for the interaction of adenosylmethionine with the cluster.86... 

The formation of this complex cofactor occurs in one of only two known reactions in which triphosphate is cleaved from ATP (Fig. 3) the other reaction is the formation of S-adenosylmethionine from ATP and methionine (see Fig. 18-18). 

Plants and bacteria produce the reduced sulfur required for the synthesis of cysteine (and methionine, described later) from environmental sulfates the pathway is shown on the right side of Figure 22-13. Sulfate is activated in two steps to produce 3-phosphoadeno-sine 5 -phosphosulfate (PAPS), which undergoes an eight-electron reduction to sulfide. The sulfide is then used in formation of cysteine from serine in a two-step pathway. Mammals synthesize cysteine from two amino acids methionine furnishes the sulfur atom and serine furnishes the carbon skeleton. Methionine is first converted to 5-adenosylmethionine (see Fig. 18-18), which can lose its methyl group to any of a number of acceptors to form A-adenosylhomocysteine (adoHcy). This demethylated product is hydrolyzed to free homocys-... 

Yu and Wang431 considered that indole-3-acetic acid exerts its stimulating effect on expansion growth by inducing the synthesis of the enzyme catalyzing the conversion of S-adenosylmethionine into ACC, a conclusion at variance with the suggestion of Vioque and coworkers432 that indoleacetic acid oxidase and its substrate (IAA) participate in the last reaction in the ethylene biosynthesis pathway, namely, the formation of ethylene from ACC. 

The role of ATP and S-adenosylmethionine in the reaction remains an intriguing but as yet unresolved question. Recently Yuan and Meselson have reported that in the presence of magnesium ion, ATP and S-adenosylmethionine the R-K endonuclease forms a specific complex with its DNA substrate (55). Complex formation is, however, observed at ATP concentrations (4 X 10-8 M) at which nucleolytic activity is not detectable. This result suggests that ATP may be involved in at least two steps (1) formation of a nonhydrolytic complex at low ATP levels and (2) formation of more stable (or more numerous) complexes and nucleolytic action at higher concentrations of ATP. The S-adenosylmethionine requirement for complex formation is in the same concentration range as observed for restriction. 

A metabolite with +16 amu is generally suspected of forming by hydrox-ylation (or by some other reaction involving the addition of oxygen). However, a metabolite with +14 amu is often suspected of forming by methylation (+CH2), not by a combination of the addition of oxygen (+16) and dehydrogenation (—2). NADPH-fortified human liver microsomes cannot catalyze the methylation of dmg candidates (such reactions are catalyzed by cytosolic enzymes in the presence of. S -adenosylmethionine). However, methylation can sometimes occur as an artifact when mass spectrometry is conducted in the presence of methanol (164), and [M + 12] adducts can form from condensation reactions with formaldehyde, which is a microsomal metabolite of methanol (165). A metabolite with +30 amu is indicative of either formation of a carboxylic acid metabolite or a combination of hydroxylation (+16) and methylation (+14). Only the former can be catalyzed by NAPDH-fortified liver microsomes. 

The major (salvage) pathways for the formation of phosphatidylcholine and ethanolamine are illustrated in Figure 19.16. Free (dietary) choline and etha-nolamine are converted to their CDP derivatives, which then react with diacyl-glycerol to form phosphatidylcholine and ethanolamine. In the lungs, another pathway forms dipalmitoyl phosphatidylcholine, a powerful surfactant. Phos-phatidylethanolamine may be methylated by S-adenosylmethionine (SAM see Chapter 20) to yield phosphatidylcholine. The reaction is catalyzed by two enzymes the first methyl group is transferred via phosphatidylethanolamine N-methyltransferase I. The other two methyl groups are transferred by phosphatidylethanolamine N-methyltransferase II. Some authorities believe that the two enzymes are identical. It has also been proposed that methylation of phospha-... 

The assay was carried out in phosphate buffer with radioactive putrescine, decarboxylated S-adenosylmethionine, and enzyme. Reactions were incubated at 37°C for 90 minutes and terminated by addition of perchloric acid. The solutions were clarified by centrifugation, and the polyamines were benzoyl-ated and extracted and then analyzed. Figure 9.55 shows the analysis of samples removed at zero time (blank) and in after 60 minutes incubation (sample) at 37°C. The appearance of radioactive spermidine is shown. The rate of product formation is shown in Figure 9.56. 

Other pyruvate-containing enzymes include aspartate -decarboxylase from Escherichia coli, the enzyme that catalyzes the formation of -alanine for the synthesis of pantothenic acid (Section 12.2.4) proline reductase from Clostridium sticklandiv, phosphatidylserine decarboxylase from E. coli and phenylalanine aminotransferase from Pseudomonas fluorescens. Phospho-pantetheinoyl cysteine decarboxylase, involved in the synthesis of coenzyme A (Section 12.2.1), and S-adenosylmethionine decarboxylase seem to be the only mammalian pyruvoyl enzymes (Snell, 1990). 

Fig. 11. Reaction scheme for (I) substrate amino acid activations and dipeptide formation, (II) racemization, and (III) N-methylation. J and E2 are symbols for enzyme activities (from peptide synthetase modules) either on the same or separate proteins. R1 and R2 are amino acid side chains, where R1 is part of the first amino acid activated by the peptide synthetase indicates the linkage between 4 -phosphopantetheinylated enzyme and the substrate or peptide intermediate. AdoMet, S-adenosylmethionine AdoHcy, S-adenosylhomocysteine...

Methionine Degradation Requires the Formation of a Key Methyl Donor, S-Adenosylmethionine... 

Figure 24.14. Activated Methyl Cycle. The methyl group of methionine is activated by the formation of S-adenosylmethionine.

Ethylene formation. Propose a mechanism for the conversion of -adenosylmethionine into 1-aminocyclopropane-l-carboxylate (ACC) by ACC synthase, a PLP enzyme. What is the other product ... 

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