Group transfer coenzymes

Coenzymes effecting transfer of groups. Examples of this class are adenosine triphosphate (ATP), biotin, coenzyme A and pyridoxal phosphate. 

FIGURE 18.29 Vitamin B19 functions as a coenzyme in intramolecular rearrangements, reduction of ribonucleotides, and methyl group transfers. 

The first two of these are mediated by 5 -deoxyadenosylcobalamin, whereas methyl transfers are effected by methylcobalamin. The mechanism of ribonucleotide reductase is discussed in Chapter 27. Methyl group transfers that employ tetrahydrofolate as a coenzyme are described later in this chapter. 

Coenzymes serve as recyclable shuttles—or group transfer reagents—that transport many substrates from their point of generation to their point of utilization. Association with the coenzyme also stabilizes substrates such as hydrogen atoms or hydride ions that are unstable in the aqueous environment of the cell. Other chemical moieties transported by coenzymes include methyl groups (folates), acyl groups (coenzyme A), and oligosaccharides (dolichol). 

A new, heat-stable, coenzyme concerned with methyl group transfer has been isolated from Methanobacterium. The coenzyme, which is involved in transmethylation reactions prior to methane formation by the organism, contains phosphorus and has a u.v. absorption at 260 nm, suggesting that it may be a nucleotide. 

Acyl-coenzyme A compounds, which, because of their high acetyl group transfer potential represent carriers of activated acyl groups in biological systems, are prepared from the corresponding carboxylic acid, CDI, and coenzyme a.[4h5],[177],[182],[183]... 

Pantothenic acid Coenzyme A Transfer of acetyl groups... 

Coenzyme Examples of chemical groups transferred Dietary precursor in mammals... 

Reactions Requiring Acyl-Group Transfers Linked to Oxidation—Reduction Biotin Mediates Carboxylations Folate Coenzymes Are Used in Reactions for One-Carbon Transfers... 

Phosphopantetheine coenzymes form thioester linkages with acyl groups, which they activate for group transfer reactions. 

Folate Coenzyme-Mediated Transfer of One-Carbon Groups Jeanne I. Rader and F. M. Huennekens... 

Figure 2.28. Coenzymes that are important in dmg metabolism. The groups transferred to the drags are shown in red. a UDP-glucuronide (transfers glucmonate), b PAPS (transfers sulfate), c S-adenosylmethionine (transfers methyl group).

In the absence of the amino acid substrate, pyridoxal phosphate is bound to enzymes by the formation of a Schiff base to the -amino group of a lysine residue at the active site. As shown in Figure 9.2, the first reaction between the substrate and the coenzyme is transfer of the aldimine linkage from the e-amino group of the lysine residue to the a-amino group of the substrate. 

The catalysis of the transfer of a methyl group is an important role of enzyme-bound vitamin B12 derivatives in human, animal, and bacterial metabolism. The known enzyme-controlled methyl group-transfer reactions are key steps in the cobamide-dependent methylations of homocysteine to methionine, in the metabolic formation of methane from other Ci-compounds in methanogenic bacteria, and in the fixation of carbon dioxide via the acetyl coenzyme A pathway of some bacterial autotrophs (Figure 10). ... 

In addition to serving as structural motifs, enols and enolates are involved in diverse biological processes. Several enol/enolate intermediates have been proposed to be involved in glycolysis (Section IV.A), wherein c/ -enediol 21 is proposed to be an intermediate in the catalytic mechanism of phosphohexose isomerase and an enol-containing enamine intermediate (22) has been proposed in the catalytic pathway of class I aldolase. In the case of glucose-fructose (aldose-ketose) isomerization, removal of the proton on Cl-OH produces the aldose while deprotonation of C2-OH yields the ketose, which is accompanied by protonation at the C2 and Cl positions, respectively. There are several cofactors that are involved in various biological reactions, such as NAD(H)/NADP(H) in redox reaction and coenzyme A in group transfer reactions. Pyridoxal phosphate (PLP, 23) is a widely distributed enzyme cofactor involved in the formation of a-keto acids, L/D-amino... 

Another purine-containing molecule of outstanding biological importance is acctyl-coenzyme A (6). Due to the high group transfer potential of acetyl groups it plays a key role in a number of metabolic reactions in which units are transferred. 

Pantothenic acid Coenzyme A Acyl-group transfer Hypertension... 

Reduction of 7/, A °-methylene-H4MPT to A( -methyl-H4MPT is catalyzed by methylene-H4MPT reductase (Reaction 5, Table 2). Reduced coenzyme factor F420 is the physiological electron donor. The enzyme appears to be soluble and not to contain a prosthetic group [67-69a]. The subsequent methyl-group transfer from... 

Methyl-H4MPT H-S-CoM methyltransferase catalyzes methyl-group transfer from methyl-tetrahydromethanopterin to coenzyme M ... 

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