Methionine, biosynthesis

Methyltetrahydrofohc acid folate) (4) is involved in methionine biosynthesis. Condensation of formaldehyde with folate (3),... 

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. 

HESSE, H., KREFT, O., MAIMANN, S ZEH, M., WILLMITZER, L., HOFGEN, R Approaches towards understanding methionine biosynthesis in higher plants, Amino Acids, 2001,20,281-289. 

A7. Anonymous, Methylated THF aids methionine biosynthesis. Chem. Eng. News 40, 36-37 (1961). 

Patton, E. E., Willems, A. R., Sa, D., Kuras, L, Thomas, D., Craig, K. L, and Tyers, M. Cdc53 is a scaffold protein for multiple Cdc34/Skpl/F-box protein complexes that regulate cell division and methionine biosynthesis in yeast. Genes Dev. 1998, 32, 692-705. 

If a methionine auxotroph is not used, the methionine within the culture can be replaced by SeMet using a methionine biosynthesis inhibition or poisoning method such as that outlined in Protocol 2.7. This method has the advantage of using any E. coli strain available and thus the initial growth of the culture is not reduced, however the method is more laborious than using the methionine auxotroph. 

However, pyrimethanil and mepanipyrim do not inhibit proteinase, cellulase or polygalacturnase activity in Botrytis cinerea17 but reduce pectinase and invertase secretion with an associated increase in their intracellular accumulation. This is proposed to be the mechanism of action of the anilinopyrimidines but the biochemical basis of the effect is not known. There is evidence that suggests the involvement of methionine biosynthesis inhibition.18... 

P. Masner, P. Muster and J. Schmid, Possible Methionine Biosynthesis Inhibition by Pyrimidinamine Fungicides , Pesticide Sci., 1994, 42, 163— 166. 

S. K. Shapiro and F. Schlenck, eds., Transmethylation and Methionine Biosynthesis. Univ. of Chicago Press, Chicago, Illinois, 1965. 

Vitamin B12 activates methyl groups for methionine biosynthesis by binding them to the Co ion at the sixth position. The methyl group donor to Bi2 is 5-methyl tetrahydrofolate. The methyl-Bi2 donates its methyl group to homocysteine, forming methionine. 

Production and Inhibition of Ethylene. Now I would like to illustrate how knowledge about a plant hormone can be used to control and regulate its action. Methionine is the precursor of ethylene in plant tissues (30). Therefore, any compound which blocks methionine metabolism might be expected to inhibit ethylene biosynthesis. Rhizobitoxine was recognized as an inhibitor of methionine biosynthesis (31) as were its analogues shown in Figure 6 (32). 9... 

In vitro. Reaction (18) proceeds irreversibly the analogous reaction in methionine biosynthesis is also highly exergonic [184]. In Methanosarcina strain G61 excess energy from Reaction (18) is conserved [195]. Working with washed everted vesicles of this organism Becker et al. [195] showed that the methyl transfer from CH3-H4 MPT to CoM generates a primary sodium motive force. 

Meth54tetrahydrofohc acid (5-CH2-H folate) (4) is involved in methionine biosynthesis. Condensation of formaldehyde with folate (3), followed by the reduction of the intermediate 5,10-CH2-H folate (5) with sodium borohydride gave 5-CH -H4 folate (4) (53). 

The metabolism of folic acid involves reduction of the pterin ting to different forms of tetrahydrofolylglutamate. The reduction is catalyzed by dihydtofolate reductase and NADPH functions as a hydrogen donor. The metabolic roles of the folate coenzymes are to serve as acceptors or donors of one-carbon units in a variety of reactions. These one-carbon units exist in different oxidation states and include methanol, formaldehyde, and formate. The resulting tetrahydrofolylglutamate is an enzyme cofactor in amino acid metabolism and in the biosynthesis of purine and pyrimidines (10,96). The one-carbon unit is attached at either the N-5 or N-10 position. The activated one-carbon unit of 5,10-methylene-H folate (5) is a substrate of T-synthase, an important enzyme of growing cells. 5-10-Methylene-H folate (5) is reduced to 5-methyl-H,j folate (4) and is used in methionine biosynthesis. Alternatively, it can be oxidized to 10-formyl-H folate (7) for use in the purine biosynthetic pathway. 

Cherest, H., Eichler, F. and deRobichon-Szulmajster, H., 1969. Genetic and regulatory aspects of methionine biosynthesis in Saccharomyces cerevisiae. J. Bacteriol., 97 328-336. 

Betaine functions as a methyl donor (e.g., in methionine biosynthesis from homocysteine Chapter 17), and it can also be converted to glycine. 

Cystathionine y-synthase (CGS) is a rather unique PLP-enzyme that catalyzes a transsulfuration reaction important in microbial methionine biosynthesis. It is the only known enzyme whose function is the catalysis of a PLP-dependent replacement reaction at the y-carbon of the amino acid substrate the succinyl moiety of O-succinyl-L-homoserine is replaced by i-Cys to give the thioether linkage of L,/.-cystathionine (scheme II). In the absence of L-Cys, the enzyme catalyzes a net y-elimination reaction from OSHS (scheme II). Because both reactions require the elimination of succinate, the catalytic pathways must diverge from a common reaction intermediate. It was originally hypothesized that a vinylglycine quinonoidal intermediate (structure 11)... 

Once choline has entered the cell, its normal fate is rapid phosphorylation by choline kinase (Fig. 3). In neurons choline is also converted to the neurotransmitter, acetylcholine. Choline can also be oxidized to betaine [-00C-CH2-N (CH3)3] in the liver and kidney. In liver, betaine is an important donor of methyl groups for methionine biosynthesis. Betaine is produced in mitochondria into which choline is transported by a specific transporter on the inner membranes. Next, choline is oxidized to betaine aldehyde by choline dehydrogenase on the inner leaflet of the mitochondrial inner membranes and the subsequent conversion to betaine is catalyzed by betaine-aldehyde dehydrogenase in the mitochondrial matrix. Betaine can be transported into kidney medulla by a betaine transporter. In renal medulla and many plants and organisms, betaine accumulates as an osmolyte (a small organic solute that accumulates in response to hypertonicity without adverse effects to the cell or organism) (J.S. Handler, 1992). Hypertonicity of the renal medulla is important for the kidney s ability to concentrate urine. 

Vitamin B12 is virtually nontoxic, even at high oral or injected doses excessive amounts are rapidly excreted. However, occasionally allergic responses to injected vitamin B12 occur (Fisher 1973), and adverse reactions to the combined administration of large injected doses of vitamin B12 and of oral vitamin C have been reported (Schrauzer 1979). Vitamin B12 is required for methionine biosynthesis and functions in conjunction with folic acid as the intermediate carrier of the methyl group. In its coenzyme form (5 -deoxyadenosylcobala-min), it is required for the conversion of methylmalonyl-CoA to succinyl-CoA. (Friedrich 1987). Bacteria utilize vitamin Bjj or its coenzyme in certain dehydrases, deaminases, and in methane biosynthesis. 

Strain D-60 was then used to select mutants resistant to the threonine analog, S-hydroxynorvaline, following nitrosoguanidine mutagenesis (j, ). Three kinds of resistant strain were saved for subsequent use. One of these (HNr31) had an Incomplete but undefined block in methionine biosynthesis, which may have accounted for the small amount of threonine that was accumulated from homoserine being funneled into the threonine pathway. 

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