Homocysteine methionine formation

The reactions catalyzed by B12 may be grouped into two classes those catalyzed by methylcobalamin and those catalyzed by cofactor B,2. The former reactions include formation of methionine from homocysteine, methanogenesis (formation of methylmercury is an important side reaction), and synthesis of acetate from carbon dioxide (82). The latter reactions include the ribonucleotide reductase reaction and a variety of isomerization reactions (82). Since dehydration and deamination have been studied quite extensively and very possibly proceed via [Pg.257]

The role of the S-methylmethionine-homocysteine or S-adenosylme-thionine-homocysteine methyltransferases in microorganisms or plants is far from clear. The latter activity participates in methionine formation according to Eq. (6). 

Incubations were set up as specified in Fig. 3, resulting in the consumption of more than 90 % of the substrates according to n. m. r. analysis of suitable model compounds the methionine produced was isolated by ion exchange technique and subsequently converted into its bis-trimethylsilyl (bis-TMS) derivatives upon reaction with N-(trimethylsilyl)diethylamine. Control experiments, in which the substrates, or homocysteine, were omitted from the incubations, gave no trace of methionine. A non-catalyzed methionine formation, detectable in the absence of the enzyme preparation, proceeded with a rate too low to seriously compete with the enzyme-catalyzed reaction. 

Choline, an essential nutrient for humans, is consumed in many foods. It is part of several major phospholipids (including phosphatidylcholine - also called lecithin) that are critical for normal membrane structure and function. Also, as the major precursor of betaine it is used by the kidney to maintain water balance and by the liver as a source of methyl groups for the removal of homocysteine in methionine formation. Finally, choline is used to produce the important neurotransmitter acetylcholine (catalyzed by choline acetyltransferase in cholinergic neurons and in such non-nervous tissues as the placenta). Each of these functions for choline is absolutely vital for the maintenance of normal function. 

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

Methionine synthase deficiency (cobalamin-E disease) produces homocystinuria without methylmalonic aciduria. This enzyme mediates the transfer of a methyl group from methyltetrahydrofolate to homocysteine to yield methionine (Fig. 40-4 reaction 4). A cobalamin group bound to the enzyme is converted to methylcobalamin prior to formation of methionine. 

It is the role of jV5-methyl THF which is key to understanding the involvement of cobalamin in megaloblastic anaemia. The metabolic requirement for N-methyl THF is to maintain a supply of the amino acid methionine, the precursor of S-adenosyl methionine (SAM), which is required for a number of methylation reactions. The transfer of the methyl group from jV5-methyl THF to homocysteine is cobalamin-dependent, so in B12 deficiency states, the production of SAM is reduced. Furthermore, the reaction which brings about the formation of Ns-methyl THF from N5,N10-methylene THF is irreversible and controlled by feedback inhibition by SAM. Thus, if B12 is unavailable, SAM concentration falls and Ah -methyl THF accumulates and THF cannot be re-formed. The accumulation of AT-methyl THF is sometimes referred to as the methyl trap because a functional deficiency of folate is created. 

Figure 15.5 Four reactions involved in methylatlon. The reactions are (1) formation of S-adenosylmethlonIne (SAM) (11) transfer of methyl group to an acceptor (111) conversion of S-adenosylmethlonIne to homocysteine (Iv) conversion of homocysteine to methionine using methyl tetrahydrofolate as the methyl donor with the formation of FH4.

Figure 22.7 Homocysteine formation from methionine and formation of thiolactone from homocysteine. The homocysteine concentration depends upon a balance between the activities of homocysteine methyltransferase (methionine synthase) and cystathionine p-synthase. Both these enzymes require vitamin B12, so a deficiency can lead to an increase in the plasma level of homocysteine. (For details of these reactions, see Chapter 15.) Homocysteine oxidises spontaneously to form thiolactone, which can damage cell membrane.

A simple observation led to the identification of homocysteine as a risk factor for coronary heart disease. Homocysteine is an intermediate in metabolism of the amino acid methionine. Indeed, the first reaction in the catabolism of methionine involves the formation of homocysteine but it can be converted back to methionine in a reaction that is catalysed by methionine synthase (see Figure 22.7). 

In animal metabolism, derivatives of cobalamine are mainly involved in rearrangement reactions. For example, they act as coenzymes in the conversion of methylmalonyl-CoA to succinyl-CoA (see p. 166), and in the formation of methionine from homocysteine (see p. 418). In prokaryotes, cobalamine derivatives also play a part in the reduction of ribonucleotides. 

Enzymatic O-methylation of flavonoids, which is catalyzed by O-methyltransferases (E.C. 2.1.1.6-) involves the transfer of the methyl group of an activated methyl donor, S -adenosyl-L-methionine, to the hydroxyl group of a flavonoid acceptor with the formation of the corresponding methylether and S -adenosyl-L-homocysteine. The latter product is, in... 

A deficiency of vitamin B12 causes the accumulation of homocysteine due to reduced formation of methylcobalamin, which is required for the conversion of homocysteine to methionine (Figure 33-3, section 1). The increase in serum homocysteine can be used to help establish a diagnosis of vitamin B12 deficiency (Table 33-2). There is concern that... 

There are two major disposal pathways for homocysteine. Conversion to methionine requires folate and vitamin B12-derived cofactors. The formation of cysteine requires vitamin B6(pyridoxine). 

Cysteine is formed in plants and in bacteria from sulfide and serine after the latter has been acetylated by transfer of an acetyl group from acetyl-CoA (Fig. 24-25, step f). This standard PLP-dependent (3 replacement (Chapter 14) is catalyzed by cysteine synthase (O-acetylserine sulfhydrase).446 447 A similar enzyme is used by some cells to introduce sulfide ion directly into homocysteine, via either O-succinyl homoserine or O-acetyl homoserine (Fig. 24-13). In E. coli cysteine can be converted to methionine, as outlined in Eq. lb-22 and as indicated on the right side of Fig. 24-13 by the green arrows. In animals the converse process, the conversion of methionine to cysteine (gray arrows in Fig. 24-13, also Fig. 24-16), is important. Animals are unable to incorporate sulfide directly into cysteine, and this amino acid must be either provided in the diet or formed from dietary methionine. The latter process is limited, and cysteine is an essential dietary constituent for infants. The formation of cysteine from methionine occurs via the same transsulfuration pathway as in methionine synthesis in autotrophic organisms. However, the latter use cystathionine y-synthase and P-lyase while cysteine synthesis in animals uses cystathionine P-synthase and y-lyase. 

The methylation of homocysteine to form methionine, with the concomitant formation of tetrahydrofolate from 5-methyltetrahydrofolate (167,168)... 

Although numerous enzymatic reactions requiring vitamin B12 have been described, and 10 reactions for adenosylcobalamin alone have been identified, only three pathways in man have so far been recognized, one of which has only recently been identified (PI). Two of these require the vitamin in the adenosyl form and the other in the methyl form. These cobalamin coenzymes are formed by a complex reaction sequence which results in the formation of a covalent carbon-cobalt bond between the cobalt nucleus of the vitamin and the methyl or 5 -deoxy-5 -adenosyl ligand, with resulting coenzyme specificity. Adenosylcobalamin is required in the conversion of methylmalonate to succinate (Fig. 2), while methylcobalamin is required by a B12-dependent methionine synthetase that enables the methyl group to be transferred from 5-methyltetrahydrofolate to homocysteine to form methionine (Fig. 3). 

Tetrahydrofolate functions as a carrier of one-carbon units. There are numerous metabolic reactions that require either the addition or removal of a one-carbon unit of some specific oxidation state. THF binds one-carbon units of three oxidation levels the methanol, formaldehyde, and formate states. These are shown in Table 6.4 along with their origins and uses. The various one-carbon units are interconvertible, as shown in Figure 6.5. Nicotinamide coenzymes are involved. In addition, the one-carbon unit may be released as C02. The methanol-level THF-bound one-carbon unit 5-methyl-THF is the storage and transport form. Once formed, its main pathway of metabolism is to form methionine from homocysteine, a reaction that requires vitamin B12 in the form of methylcobalamin (see Figure 6.2 and Chapter 20) ... 

This functional deficiency of folate is exacerbated by the associated low concentrations of methionine and S-adenosyl methioitine, although most tissues (apart from the central nervous system) also have betaine-homocysteine methyltransferase that may be adequate to maintain tissue pools of methionine. Under normal conditions S-adenosyl methioitine inhibits methylene-tetrahydrofolate reductase and prevents the formation of further methyl-tetrahydrofolate. Relief of this inhibition results in increased reduction of one-carbon substituted tetrahydrofolates to methyl-tetrahydrofolate. 

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