Radical succinyl

Radical, succinyl aminyl, transformation into pyrrolidines 78YGK352. Reactions of N-heterocycles in multiphase systems using fluoride anion ... 

Methylmalonyl-CoA mutase (MCM) catalyzes a radical-based transformation of methylmalonyl-CoA (MCA) to succinyl-CoA. The cofactor adenosylcobalamin (AdoCbl) serves as a radical reservoir that generates the S -deoxyadenosine radical (dAdo ) via homolysis of the Co—C5 bond [67], The mechanisms by which the enzyme stabilizes the homolysis products and achieve an observed 1012-fold rate acceleration are yet not fully understood. Co—C bond homolysis is directly kineti-cally coupled to the proceeding hydrogen atom transfer step and the products of the bond homolysis step have therefore not been experimentally characterized. 

A recent study has indicated that the skeletal rearrangement step in the B12-catalysed isomerization of methylmalonyl-CoA to succinyl-CoA occurs not by a radical pathway but by an anionic or organocobalt pathway. A computational study of the isomerization of allyl alcohol into homoallyl alcohol by lithium amide has pointed to a process proceeding via a transition state in which the proton is half transferred between carbon and nitrogen in a hetero-dimer. l,l-Dilithio-2,2-diphenylethene... 

Adenosylcobalamin (coenzyme 812) carries a covalently bound adenosyl residue at the metal atom. This is a coenzyme of various isomerases, which catalyze rearrangements following a radical mechanism. The radical arises here through homolytic cleavage of the bond between the metal and the adenosyl group. The most important reaction of this type in animal metabolism is the rearrangement of methylmalonyl-CoAto form succinyl-CoA, which completes the breakdown of odd-numbered fatty acids and of the branched amino acids valine and isoleucine (see pp. 166 and 414). 

The product of acetyl-CoA carboxylase reaction, malonyl-CoA, is reduced via malonate semialdehyde to 3-hydroxypropionate, which is further reductively converted to propionyl-CoA. Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by the same carboxylase. (S)-Methylmalonyl-CoA is isomerized to (R)-methylmal-onyl-CoA, followed by carbon rearrangement to succinyl-CoA by coenzyme B 12-dependent methylmalonyl-CoA mutase. Succinyl-CoA is further reduced to succinate semialdehyde and then to 4-hydroxybutyrate. The latter compound is converted into two acetyl-CoA molecules via 4-hydroxybutyryl-CoA dehydratase, a key enzyme of the pathway. 4-Hydroxybutyryl-CoA dehydratase is a [4Fe-4S] cluster and FAD-containing enzyme that catalyzes the elimination of water from 4-hydroxybutyryl-CoA by a ketyl radical mechanism to yield crotonyl-CoA [34]. Conversion of the latter into two molecules of acetyl-CoA proceeds via normal P-oxidation steps. Hence, the 3-hydroxypropionate/4-hydroxybutyrate cycle (as illustrated in Figure 3.5) can be divided into two parts. In the first part, acetyl-CoA and two bicarbonate molecules are transformed to succinyl-CoA, while in the second part succinyl-CoA is converted to two acetyl-CoA molecules. 

Vitamin B12 is essential for the methylmalonyl-CoAmutase reaction. Methylmalonyl-CoA mutase is required during the degradation of odd-chain fatty acids and of branched-chain amino acids. Odd-chained fatty acids lead to propionyl-CoA as the last step of P-oxida-tion. Methylmalonyl-CoA can be derived from propionyl-CoA by a carboxylase reaction similar to that of fatty acid biosynthesis. The cofactor for this carboxylation reaction is biotin, just as for acetyl-CoA carboxylase. The reaction of methylmalonyl-CoA mutase uses a free radical intermediate to insert the methyl group into the dicar-boxylic acid chain. The product is succinyl-CoA, a Krebs cycle intermediate. The catabolisms of branched-chain lipids and of the branched-chain amino acids also require the methylmalonyl-CoA mutase, because these pathways also generate propionyl-CoA. 

This enzyme s role in humans is to assist the detoxification of propionate derived from the degradation of the amino acids methionine, threonine, valine, and isoleucine. Propionyl-CoA is carboxylated to (5 )-methylmalonyl-CoA, which is epimerized to the (i )-isomer. Coenzyme Bi2-dependent methylmalonyl-CoA mutase isomerizes the latter to succinyl-CoA (Fig. 2), which enters the Krebs cycle. Methylmalonyl-CoA mutase was the first coenzyme B -dependent enzyme to be characterized crystallographically (by Philip Evans and Peter Leadlay). A mechanism for the catalytic reaction based on ab initio molecular orbital calculations invoked a partial protonation of the oxygen atom of the substrate thioester carbonyl group that facilitated formation of an oxycyclopropyl intermediate, which connects the substrate-derived and product-related radicals (14). The partial protonation was supposed to be provided by the hydrogen bonding of this carbonyl to His 244, which was inferred from the crystal structure of the protein. The ability of the substrate and product radicals to interconvert even in the absence of the enzyme was demonstrated by model studies (15). 

Figure 22.15. Formation of Succinyl CoA by a Rearrangement Reaction. A free radical abstracts a hydrogen atom in the rearrangement of methylmalonyl CoA to succinyl CoA.

CoA thioesters are also the products of the oxidative decarboxylation reactions of a-keto acids, especially pyruvate and a-ketoglutarate, from which acetyl-CoA and succinyl-CoA are formed, respectively (Equation (14)). Three distinct types of enzymes catalyze such reactions however, the mechanistic involvement of CoA is generally rather limited for two of these, and only a brief discussion of each will be provided here. For more detailed information on these enzymes, the reader is referred to the relevant chapters on thiamin and lipoic acid enzymology and on radical enzymes in this series (see Chapters 1.08 and 7.03). 

The 5 -deoxyadenosyl radical from coenzyme B12 initiates the reaction by abstraction of hydrogen from the methyl group of methylmalonyl-CoA. The resultant free radical undergoes isomerization by internal cyclization to the oxycyclopropyl radical, which opens to the succinyl-CoA radical, as in the chemical counterpart of Figure 24. Hydrogen abstraction from the methyl group of 5 -deoxyadenosine completes the mechanistic cycle. 

Fig. 17 Methylmalonyl-CoA mutase (MMCM) interconverts (fi)-methylmalonyl-CoA and succinyl-CoA. Proposed reaction mechanism of the carbon skeleton rearrangement, catalyzed by MMCM involving H-atom abstraction (step a), radical rearrangement (step b) and back transfer of H-atom (step c). (The experimentally supported substrate triggered formation of the 5 -deoxy-5 -adenos)d radical and of cob(II)alamin (23, B r) by homolysis of protein bound AdoCbl (2) is omitted here, see Fig. 16 [120,173,191])...

In humans, methylmalonyl-CoA mutase is required for the metabolism of proprionate, derived from branched-chain amino acids, odd-chain fatty acids and cholesterol, into succinyl-CoA (Banerjee 1997 Pett et at. 2002). Methylmalonyl-CoA mutase requires AdoCbl as a cofactor. The mechanism involves the homolytic cleavage of the Co-C bond, forming cob(II)alamin and a 5 -deoxyadenosyl radical (Pett et at. 2002). The homolytic cleavage of the Co-C bond is increased 10 -fold in the presence of the enzyme. The 5 -deox-yadenosyl radical first abstracts a hydrogen atom from the substrate methyl-malonyl-CoA, which donates after a rearrangement reaction back to form succinyl-CoA (Pett et at. 2002). In humans, deficiency in methylmalonyl-CoA mutase causes an inherited metabolic disorder and is one of causes of methylmalonylacidemia. According to the severity of methylmalonyl-CoA mutase reduced activity, the deficiency is characterized as muf (detectable... 

CoA functions in any reaction in which an acetyl group (-CH3CO) is formed or is transferred from one substance to another. In addition to acetyl, other acyl radicals require coenzyme A it is required whenever succinyl, benzyl, or fatty acid radicals are formed or transferred. 

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