Homocysteine biosynthesis enzymes

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. 

S. cerevisiae converts inorganic Se to SeMet and incorporates it into the cellular protein in place of Met. The biosynthesis of SeMet proceeds via SeCys in analogy to that of Met, as was demonstrated in a study of a mutant strain of yeast requiring Met for growth due to a lack of homocysteine methyl transferase activity. When grown in Se-containing media, this strain produces SeCys, but no SeMet (Mason, 1994). While most of the SeCys is synthesized without involving Se-specific enzymes, recent studies indicate that some of the SeCys is also produced by a specific tRNA and incorporated into a 25 kDa... 

The coenzyme form of pantothenic acid is coenzyme A and is represented as CoASH. The thiol group acts as a carrier of acyl group. It is an important coenzyme involved in fatty acid oxidation, pyruvate oxidation and is also biosynthesis of terpenes. The epsilon amino group of lysine in carboxylase enzymes combines with the carboxyl carrier protein (BCCP or biocytin) and serve as an intermediate carrier of C02. Acetyl CoA pyruvate and propionyl carboxylayse require the participation of BCCP. The coenzyme form of folic acid is tetrahydro folic acid. It is associated with one carbon metabolism. The oxidised and reduced forms of lipoic acid function as coenzyme in pyruvate and a-ketoglutarate dehydrogenase complexes. The 5-deoxy adenosyl and methyl cobalamins function as coenzyme forms of vitamin B12. Methyl cobalamin is involved in the conversion of homocysteine to methionine. 

Other examples of PLP-requiring enzymes are the amino acid decarboxylases that lead to formation of amines, including several that are functional in nervous tissue (e.g., epinephrine, norepinephrine, serotonin, and y-aminobutyrate) cysteine desulfhydrase and serine hydroxymethyltransferase, which use PLP to effect the loss or transfer of amino acid side chains phosphorylase, which catalyzes phosphorolysis of the a-1,4-linkages of glycogen and cystathione beta-synthase in the transsulfiiration pathway of homocysteine. Additionally the biosynthesis of heme depends on the early... 

CGS catalyzes the 7-replacement reaction of an activated form of L-homoserine with L-cysteine, leading to cystathionine. 0-Succinyl-L-homoserine (l-OSHS), 0-acetyl-L-homoserine (OAHS), and 0-phospho-L-homoserine (OPHS) are substrates for CGS ftom bacteria, fungi, and plants, respectively. The plant enzyme is also able to convert the microbial substrates, albeit at much higher values. This reaction is the first step in the transsulfuration pathway that converts L-Cys into L-homocysteine, the immediate precursor of L-methionine. The 0-activated L-homoserine substrate is situated at a metabolic branch point between L-Met and L-Thr biosynthesis, and which substrate is used by CGS depends on the species. In analogy with TS, CGS is tightly regulated by SAM concentration in plants. ... 

Fig. 7. Biosynthesis of choline plasmalogens (plasmenylcholines) via modification of the sn-3 polar head group of ethanolamine plasmalogens (plasmenylethanolamines). These reactions are proposed to be catalyzed directly by (1) a base exchange enzyme or (II) At-methyltransferase. A combination of other enzymatic reactions could also result in replacement of the ethanolamine moiety of plasmenylethanolamine to produce plasmenylcholines the enzymes responsible include (IB) phospholipase C, (IV) the reverse reaction of ethanolamine phosphotransferase, (V) phospholipase D, (VI) phosphohydtolase, and (VII) cholinephosphotransferase. AdoMet, 5-adenosyl-L-methionine AdoHcy, 5-adenosyl-L-homocysteine Etn, ethanolamine GPE, sn-glycero-...

Cystathionine-y-synthase isolated from Salmonella typhimurium is a tetramer (molecular weight 160000) and catalyses, in vivo, the y-replacement of O-suc-cinylhomoserine with cysteine [79] to yield cystathionine. The latter, by way of homocysteine, is involved in the biosynthesis of methionine. In other species of bacteria and plants the succinyl moiety may be replaced by acetyl, phosphoryl, or malonyl moieties [80]. In the absence of cysteine the enzyme catalyses an abnormal reaction resulting in the formation of a-oxobutyrate. The latter reaction has been utilised for mechanistic investigations pertinent to the y-eUmination-deamination process (vide infra). 

Methyl transfer reactions play a significant part in the modifications of aromatic polyketides, both of the polyketide core [61,62] as well as of several of the sugar moieties [44,53]. In Streptomyces, more than 20 amino acid sequences have been found that may represent enzymes involved in methyl transfer reactions in the biosynthesis of aromatic polyketides [149]. One of these enzymes, the S-adenosyl-L-methionine-dependent DnrK, is involved in the methylation of the C-4 hydroxyl group in daunorubicin/doxorubicin biosynthesis (Scheme 10, step 12). The subunit of the homo-dimeric enzyme displays a fold typical for small-molecule methyltransferases. The structure of the ternary complex with bound products S-adenosyl-L-homocysteine and 4-methoxy-8-rhodomycin provided insights into the structural basis of substrate recognition and catalysis [149]. The position and orientation of the substrates suggest an Sn2 mechanism for methyl transfer, and mutagenesis experiments show that there is no catalytic base in the vicinity of the substrate. Rate enhancement is thus most likely due to orientational and proximity effects [149]. 

The biosynthesis of simple amines appears to involve a single enzyme with broad specificity. An enzyme with this activity, isolated from red algae (Rhodophyceae), was capable of decarboxylation of the L-forms of leucine, valine, isoleucine, norvaline, 2-aminobutyric acid, phenylalanine, methionine, cysteine, and homocysteine. A number of other amino acids were unable to serve as precursors (Smith, 1980). 

Coenzymes Bjj as cofactors catalyse two completely different types of reactions. Methylcobalamine (as with folacin) acts in some transmethylation reactions (such as biosynthesis of methionine from homocysteine), collaborates with foUc acid in the synthesis of DNA and red blood cells (biosynthesis of porphyrins) and in the fixation of carbon dioxide by some anaerobic acetogennic microorganisms. Enzymes using S -deoxy-S -adenosylcobalamine catalyse a number of isomerisations that are otherwise only viable with difficulty (1,2-rearrangements, such as the formation of succinyl-CoA from methyhnalonyl-CoA) and in some organisms they reduce ribonucleotides to deoxyiibonucleotides. 

Caffeine synthase, the majority of SAH hydrolase activity, and parts of the adenine-salvage pathway are localized to chloroplasts. In coffee SAM synthase is confined to the cytosol and SAM synthase genes fi-om tobacco and parsley lack a transit peptide. However, SAM synthase from tea is a chloroplastic enzyme, encoded by a nuclear gene (Koshiishi et al. 2001). The proposed model for the subcellular localization of caffeine biosynthesis begins with the production of homocysteine and its conversion to methionine in the chloroplasts. Methionine is then converted to SAM in the cytosol and transported back into the chloroplast to serve as the methyl donor in caffeine biosynthesis. Purine alkaloids are stored in vacuoles where they are thought to form complexes with chlorogenic acids (Mosli-Waldhauser and Baumann 1996). 

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