Cobalamin cofactor

Co within all compounds of the so-called cobalamin (or B12) family. The biological functions of cobalamin cofactors are defined by their axial substituents either a methyl or an adenosyl group. Both cofactors participate in biosynthesis the former in methyl transfer reactions while the latter is a free radical initiator, abstracting H atoms from substrates. Decades after their initial characterization, the fascination with the biological chemistry of cobalamins remains.1109... 

The mechanistic and structural chemistry of B12 may be separated into (i) investigations of cobalamin cofactors both apart from and in complex with their enzymes, and (ii) biomimetic model complexes, both structural and functional. 

Vitamin B12 Cobalamin Methylcobalamin Deoxyadenosyl cobalamin Cofactor for reactions Homocysteine > Methionine I Methylmalonyl CoA -> Succinyl CoA j... 

As illustrated in Fig. 1, methionine synthase is positioned at the intersection between transsulfuration and methylation pathways. As a consequence, its level of activity exerts control over cellular redox status, since it determines the proportion of HCY that will be diverted toward cysteine and GSH synthesis. Methionine synthase activity is exceptionally sensitive to inhibition during oxidative stress, primarily because its cobalamin cofactor is easily oxidized (Liptak and Brunold, 2006). This allows methionine synthase to serve as a redox sensor, lowering its activity whenever the level of oxidation increases, until increased GSH synthesis brings the system back into balance. Electrophilic compounds, such as oxygen-containing xenobiotic metabolites, also react with cobalamin, inactivating the enzyme and increasing diversion of HCY toward GSH synthesis (Watson et al., 2004). Thus, methionine synthase is a sensor of both redox and xenobiotic status. 

Many enzymes are initially synthesized as inactive forms, which require exogenous cofactors for their activity. Only after the inactive apoprotein combines with the cofactor it becomes the active holoenzyme. Cofactors may be dissociable or tightly bound with the latter often referred to as a prosthetic group. A cofactor may be a metal (e.g., iron or copper), an organic compound (e.g., pyridoxal phosphate or flavin), or an organometallic compound (e.g., heme or cobalamin). Cofactors are necessary to assist amino acid residues at the active sites... 

The presence of a carbon in the core of the ring indicates that these macrocycles will form organometallic interactions upon metal binding. In biological systems, naturally occurring organometallic complexes are rare. The cobalamin cofactor is one of the more important examples, which possesses an axial metal-carbon bond in a cobalt porphyrinoid macrocycle (77). Another common example is seen in carbon-monoxide deactivated ferrous heme (22) ... 

Plants catalyze the final step in the de novo synthesis of methionine by means of tetrahydropteroyltriglutamate methyltransferase (E.C. 2.1.1.14), a cobalamin-independent enzyme that uses the triglutamyl derivative of N -methyltetrahydrofolic acid at a faster rate than the monoglutamyl derivative. In contrast, an enzyme from animals and from certain bacteria utilizes the monoglutamyl derivative at least as rapidly as the triglutamyl, contains a cobalamin cofactor, and may require catalytic amounts of AdoMet (for detailed information see Cossins, this series, Vol. 2, Chapter 9). 

Figure 1. The cobalamin cofactor in its alkyl Co(III) form. The alkyl substituent is a methyl group in MetH and other methyltransferases, and is 5 deoxyadenosine in Bj2 dependent mutases.

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. 

Figure 5. The structure of the activation complex (left) with the activation domain enclosing the cobalamin and part of the Bj binding domain, and the cap domain at the lower right. The cobalamin cofactor, with its side chain protruding into the B 12-binding domain, is shown in ball-and-stick mode. AdoMet has been included at the site where it binds in the isolated activation domain 4). From cross-linking experiments 49), it is known that flavodoxin binds to this face of the gactivation complex. On the right is a model for the structure of the [649-1227] fragment in its cap-on conformation. The motions of the activation and cap domains that occur in the interconversion of conformations involve rotations around axes that are not parallel to one another.

The sulfur amino acids are methionine, homocyst(e)ine, cystathionine, cyst(e)ine, and taurine. Defects in several of the enzymatic steps of their metabolism are known some, but not all, result in human disease. The re-methylation of homocysteine to methionine is closely dependent on folate and cobalamin cofactors, and relevant defects of their metabolism are therefore included in this chapter. Cystinuria and cystinosis, defects of renal tubular and lysosomal transport of cystine, respectively, are described in Chap. 13. 

The deficiencies of cystathionine )5-synthase (CBS), sulfite oxidase, and methylenetetrahydrofolate reductase (MTHFR) may all result in central nervous system dysfunction, in particular mental retardation [1-3]. Defects of CBS and sulfite oxidase both cause dislocated lenses of the eyes, but the phenotypes are different otherwise. The manifestations of CBS deficiency, the most common of these disorders, and MTHFR deficiency range from severely affected to asymptomatic patients both may cause vascular occlusion. Deficiency of sulfite oxidase is clinically uniform, but genetically heterogeneous, and functional deficiency of the enzyme can result from several inherited defects of molybdenum cofactor biosynthesis [2, 4]. Hereditary folate malabsorption and defects of cobalamin transport (transcobala-min II deficiency) or cobalamin cofactor biosynthesis (cblC-G diseases) may cause megaloblastic anemia, in addition to CNS dysfunction [3, 5, 6]. 

Amberlite XAD adsorption has been applied by Koppenhagen et al. (57) for the isolation of rhodium analogs of cobalamin cofactors. Gimsing et al. (61) used it for the desalting of chemically prepared radioactive cobalamin forms, and Stupperich et al. (62) extracted cobamides from various bacterial species with this material. 

Riedel, B., T. Fiskerstrand, H. Refsum and P.M. Ueland, 1999. Co-ordinate variations in methylmalonyl-CoAmutase, and the cobalamin cofactors in human glioma cells during nitrous oxide exposure and the subsequent recovery phase. Biochem. J. 341, 133-138. 

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