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Enzymes Cobalamine-catalyzed reactions

Perhaps the best-characterized example of this mechanism involves the synthesis of heme cofactors and their subsequent incorporation into various hemoproteins (see Iron Heme Proteins Electron Transport). Succinctly, enzyme-catalyzed reactions convert either succinyl-CoA or glutamate into 5-ammolevulinic acid. This molecule is further converted through a series of intermediates to form protoporphyrin IX, the metal-ffee cofactor, into which Fe is inserted by ferrochelatase. Analogous reactions are required for the synthesis of other tetrapyrrole macrocycles such as the cobalamins (see Cobalt Bu Enzymes Coenzymes), various types of chlorophylls, and the methanogen coenzyme F430 (containing Co, Mg, or Ni, respectively). Co- and Mg-chelatases have been described for insertion of these metals into the appropriate tetrapyrrolic ring structures. ... [Pg.5512]

In the absence of oxygen the photodecomposition of adenosylcobalamin leads to the formation of Co +-cobalamin (22) and a 5 -deoxyadenosyl that cy-clizes to 8,5-cyclic-adenosine (23). In the presence of oxygen, aquocobalamin and adenosine-5 -carboxaldehyde are formed (24). Photolysis of methylcobala-min occurs very rapidly in aqueous solution with formation of formaldehyde and aquocobalamin as the major products. In the absence of oxygen the reaction is rather slow and gives rise to the formation of Co -cobalamin and methane (25,26). Remarkably, photolysis of methylcobalamin in the presence of homocysteine yields methionine, a methylation reaction that under aerobic, intracellular conditions occurs only in an enzyme-catalyzed reaction with reductive activity... [Pg.520]

In mammals and in the majority of bacteria, cobalamin regulates DNA synthesis indirectly through its effect on a step in folate metabolism, catalyzing the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate via two methyl transfer reactions. This cytoplasmic reaction is catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyl-transferase), which requires methyl cobalamin (MeCbl) (253), one of the two known coenzyme forms of the complex, as its cofactor. 5 -Deoxyadenosyl cobalamin (AdoCbl) (254), the other coenzyme form of cobalamin, occurs within mitochondria. This compound is a cofactor for the enzyme methylmalonyl-CoA mutase, which is responsible for the conversion of T-methylmalonyl CoA to succinyl CoA. This reaction is involved in the metabolism of odd chain fatty acids via propionic acid, as well as amino acids isoleucine, methionine, threonine, and valine. [Pg.100]

Naturally, the biosynthesis of cobalamins themselves require delivery of Co ions at a particular point in the reaction scheme. Cobaltochelatase catalyzes the ATP-dependent insertion of Co11 into the corrin ring during the biosynthesis of coenzyme B12 in Pseudomonas denitrifleans. Cobaltochelatase is a heterodimeric enzyme (140 KDA and 450 KDA subunits each inactive in isolation), and the two components have been isolated and purified to homogeneity.1119 The reaction product is divalent cobyrinic acid, demonstrating that hydrogenobyrinic acid and its diamide (255) are precursors of AdoCbl. [Pg.101]

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. [Pg.220]

This cobalamin-dependent enzyme [EC 2.1.1.13], also known as methionine synthase and tetrahydropteroyl-glutamate methyltransferase, catalyzes the reaction of 5-methyltetrahydrofolate with L-homocysteine to produce tetrahydrofolate and L-methionine. Interestingly, the bacterial enzyme is reported to require 5-adenosyl-L-methionine and FADH2. See also Tetrahydropteroyl-triglutamate Methyltransferase... [Pg.462]

This cobalamin-dependent enzyme catalyzes the reaction of methyltetrahydromethanopterin with coenzyme M to produce methyl-coenzyme M and tetrahydrometha-nopterin. [Pg.462]

Figure 21-3. The methionine synthase reaction. Methionine synthase catalyzes the remethylation of homocysteine to methionine. In the first half reaction (1), a methyl group is transferred from 5-methyl tetrahydrofolate (5-MTHF) to the reduced form of cobalamin [Cob(I)], generating methyl-cobalamin [Methyl-Cob(III)] and tetrahydrofolate (THF). During the second half reaction (2), the methyl group is transferred from methylcobalamin to homocysteine, generating methionine. During the catalytic reaction, Cob(I) occasionally becomes oxidized, producing an inactive form of cobalamin, cob(II)alamin [Cob(II)]. The enzyme methionine synthase reductase (MTRR) then reactivates Cob(II) through reductive methylation, producing methyl-Cob(III). SAM, 5-adenosylmethionine SAH, 5-adeno-sylhomocysteine. Figure 21-3. The methionine synthase reaction. Methionine synthase catalyzes the remethylation of homocysteine to methionine. In the first half reaction (1), a methyl group is transferred from 5-methyl tetrahydrofolate (5-MTHF) to the reduced form of cobalamin [Cob(I)], generating methyl-cobalamin [Methyl-Cob(III)] and tetrahydrofolate (THF). During the second half reaction (2), the methyl group is transferred from methylcobalamin to homocysteine, generating methionine. During the catalytic reaction, Cob(I) occasionally becomes oxidized, producing an inactive form of cobalamin, cob(II)alamin [Cob(II)]. The enzyme methionine synthase reductase (MTRR) then reactivates Cob(II) through reductive methylation, producing methyl-Cob(III). SAM, 5-adenosylmethionine SAH, 5-adeno-sylhomocysteine.
ATP to yield the d isomer of methylmalonyl CoA (Figure 22.11). This carboxylation reaction is catalyzed by propionyl CoA carboxylase, a biotin enzyme that is homologous to and has a catalytic mechanism like that of pyruvate carboxylase (Section 16.3.2). The d isomer of methylmalonyl CoA is racemized to the 1 isomer, the substrate for a mutase that converts it into succinyl CoA by an intramolecular rearrangement. The -CO-S-CoA group migrates from C-2 to C-3 in exchange for a hydrogen atom. This very unusual isomerization is catalyzed by methylmalonyl CoA mutase, which contains a derivative of vitamin Bj2, cobalamin, as its coenzyme. [Pg.911]

Cobalamin enzymes, which are present in most organisms, catalyze three types of reactions (1) intramolecular rearrangements (2) methylations, as in the synthesis of methionine (Section 24.2.7) and (3) reduction of ribonucleotides to deoxyribonucleotides (Section 25.3). In mammals, the conversion of 1-methylmalonyl CoA into succinyl CoA and the formation of methionine by methylation of homocysteine are the only reactions that are known to require coenzyme Bj2. The latter reaction is especially important because methionine is required for the generation of coenzymes that participate in the synthesis of purines and thymine, which are needed for nucleic acid synthesis. [Pg.911]

Figure 22.13. Rearrangement Reaction Catalyzed by Cobalamin Enzymes. The R group can be an amino group, a hydroxyl group, or a substituted carbon. Figure 22.13. Rearrangement Reaction Catalyzed by Cobalamin Enzymes. The R group can be an amino group, a hydroxyl group, or a substituted carbon.
Homocysteine is metabolized in the liver, kidney, small intestine and pancreas also by the transsulfuration pathway [1,3,89]. It is condensed with serine to form cystathione in an irreversible reaction catalyzed by a vitamin B6-dependent enzyme, cystathionine-synthase. Cystathione is hydrolyzed to cysteine that can be incorporated into glutathione or further metabolized to sulfate and taurine [1,3,89]. The transsulfuration pathway enzymes are pyridoxal-5-phosphate dependent [3,91]. This co-enzyme is the active form of pyridoxine. So, either folates, cobalamin, and pyridoxine are essential to keep normal homocysteine metabolism. The former two are coenzymes for the methylation pathway, the last one is coenzyme for the transsulfuration pathway [ 1,3,89,91 ]. [Pg.145]

Methylcobalamia is iavolved ia a critically important physiological transformation, namely the methylation of homocysteine (8) to methionine (9) (eq. 2) catalyzed by A/ -methyltetrahydrofolate homocysteine methjitransferase. The reaction sequence involves transfer of a methji group first from A/5 -methjltetrahydrofolate to cobalamin (yielding methjicobalamin) and thence to homocysteine. Once again, the intimate details of the reaction are not weU known (31). Demethylation of tetrahydrofolate to tetrahydrofohc acid is a step in the formation of thymidine phosphate, in turn requited for DNA synthesis. In the absence of the enzyme, excess RNA builds up in ted blood cells. [Pg.112]

Figure 22.14 Rearrangement reaction catalyzed by cobalamin enzymes. The R... Figure 22.14 Rearrangement reaction catalyzed by cobalamin enzymes. The R...
There are two classes of methionine synthases, the cobalamin-dependent and cobalamin-independent enzymes. Both synthases catalyze the same reaction, Equation (20), the methylation of (5)-homocysteine (homocysteine) by -methyltetrahydrofolate (methyltetrahydrofolate). [Pg.538]

All known ACS enzymes are bifunctional in that they possess a C cluster with COdFI activity in addition to an A cluster (the ACS active site. Scheme 9). In the enzymes, a CO tunnel is described through which GO can pass directly from the C cluster, where it is generated from CO2, to the A cluster, where acetyl GoA synthesis takes place. Again, two mechanisms were proposed that differ in the order of binding events and redox states involved. In essence, however, GO binds to an Ni-GHs species, followed by insertion and generation of an Ni-acetyl species, which upon reaction with GoA liberates the acetyl GoA product. It is interesting to note that methylation of Ni occurs by reaction with methyl cobalamin (Scheme 7). In M. thermoacetica, the cobalamin is the cofactor for a rather unique protein called the corrinoid iron sulfur protein (GFeSP). The above process, even if mechanistic details still remain in question, resembles the industrial Monsanto acetic acid synthesis process (Scheme 9, bottom). In this case, however, the reaction is catalyzed by a low-valent Rh catalyst. [Pg.890]

A further enzyme, the P-methylation enzyme, adds the further methyl group that results in the formation of the second C-P bond in phosphinothricin. The methyl donor for this reaction is methyl cobalamine [39]. The addition of two alanyl residues to phosphinothricin to form bialaphos in Streptomyces virido-chromogenes is catalyzed by synthetase enzymes, and proceeds independently of ribosomes [40]. [Pg.141]

Since the coenzyme from vitamin is required in two distinct enzyme reactions, i.e., remethylation of homocystine and catabolism of methylmalonic acid, the fundamental defect must involve a step in converting to its coenzymes. Formation of both deoxyadenosyl B and methyl B requires a prior reductive step catalyzed by cobalamin reductase, which appears to be the defective enzyme in this variant (Hogervorst et al., 2002) (Fig. 20.4). [Pg.420]

The primary reaction catalyzed by methionine synthase converts homocysteine (Hey) and methyltetrahydrofolate (CH3H4folate) to methionine and tetrahydrofolate (Figure 2). Occasional oxidation of the reactive cob(I)alamin intermediate produces an inactive cob(II)alamin enzyme, which is reactivated by a reductive methylation that uses S-adenosylmethionine (AdoMet) as the methyl donor and flavodoxin or a flavodoxin-like domain as an electron donor. Thus methionine synthase supports three distinct methyl transfer reactions each involving the cobalamin cofactor. [Pg.187]


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See also in sourсe #XX -- [ Pg.2 , Pg.2 , Pg.8 , Pg.14 , Pg.16 ]

See also in sourсe #XX -- [ Pg.2 , Pg.2 , Pg.8 , Pg.14 ]




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Cobalamine

Cobalamines

Cobalamins

Cobalamins enzymes

Enzyme-catalyzed

Enzyme-catalyzed reactions

Enzymes catalyze

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