Corrinoids, conformation

The most exciting research is yet to be performed on 13C NMR of the corrin enzymes. This could be accomplished by biosynthesis of C-13 enriched samples of biologically active B12 derivatives followed by their incorporation into enzymes. Since it has been shown that 13C-spectra of corrinoids are well resolved, and sensitive to small changes in the molecular conformation, then one could hope to get quite detailed information pertinent to the binding of B12 and to the mechnism of enzyme catalysis. 

Intrinsic factor binds the various vitamin B12 vitamers with equal affinity, but not other corrinoids. There is one vitamin B12 binding site per mole of intrinsic factor. On binding the vitamin, the protein undergoes a conformational change, resulting in dimerization and greatly enhanced resistance to proteolysis. 

The vitamin B12 and related coenzymes (cobalamins, cobinamides, and corrinoids) have been studied in detail using H and shift and relaxation studies. (595-598) With the aid of lanthanide probes the eonformations of cobalamins in solution are shown to be very similar to those in the crystalline state. The appreciable temperature dependences of the electronic and H NMR spectra are attributed to a conformational change rather than to a 6-eoordinate-5-coordinate equilibrium as previously suggested. (599) The binding of 5 -ATP to the respiratory protein hemoeyanin has been studied by H and NMR. 

Studies on protein-free corrinoids and model complexes have shown that increasing the steric bulkiness around the coordinated Ca atom can cause a dramatic labilization of the Co—C bond. The protein-coenzyme adduct might contain the coenzyme in a resting state and the protein in a strained state the substrate would then switch the system into a strained coenzyme and a relaxed enzyme with little thermodynamic barrier. The strained form of the coenzyme is then in labile equilibrium with base-on cobalt(II) and the free radical. " This hypothesis, that conformational changes in cobalamin can switch chemical reactions on and off, is closely analogous with the known aspects of hemoglobin function. 

Crystallographic studies with GM from Cl. cochlearium have provided a detailed structural picture of the enzyme, in which the corrinoid cofactor is bound base-off/His-on again and at the interface between the subunits a and The cobalt-coordinating histidine is part of an H-bonded regulatory His-Asp-Ser triad. Detailed analysis of GM with the bound coenzyme B12 (3) revealed the position of the ribose part of the 5 -deoxyadenosyl moiety to be disordered and to be present in two conformations, related to each other by a pseudo-rotation of the furanose ring. One of these structures places the 5 -methylene group of the adenosine close to the position of the corrin-bound cobalt center, but at a distance of about 3.1 A, and thus appears to have features expected for the direct product of the homolysis of the Co-C bond of the bound cofactor (3). In the other conformation, the 5 methylene carbon is at a distance of about 4.5 A from the metal center and is displaced toward the substrate-binding site, as if in van der Waals contact with the bound substrate. In this way, GM... 

Zhao, S. Y Ragsdale, S. W. A conformational change in the methyltransferase from Clostridium thermoaceticum facilitates the methyl transfer from (6S)-methyltetrahydro-folate to the corrinoid iron-sulfur protein in the acetyl-CoA pathway. Biochemistry, 1996, 35(7), 2476-2481. 

In a catalytic cycle of B -dependent methyl transferases the corrinoid is indicated to cycle between a methyl-Co(III)-corrin and a Co(I)-corrin [125,126, 153,155]. The changing between the hexacoordinate methyl-Co(III)-form and (presumably) tetracoordinate Co(I)-form is accompanied by constitu-tional/conformational changes, which are highly likely to provide a means for controlling the organometallic reactivity of the bound cofactor [170], sub-... 

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