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By Butler P.A., Krautler B.

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Tetanomorphum and Cl. cochlearium consist of a pair of B12 -binding subunits (σ ) and a larger substrate binding unit (ε). Two molecules of adenosyl-cobamide are bound in the heterotetramers (ε2 τ2 ). The cofactors from the two Clostridia were identified as pseudocoenzyme B12 (37) [151, 201] and adenosyl-factor A (38) [151], but coenzyme B12 (2) also functions as a cofactor [175, 200]. Binding of the substrate triggers homolysis of the Co – C bond of the adenosyl cofactor and in a similar situation to MMCM, homolysis and H-atom abstraction of the bound substrate are kinetically coupled.

To complete the catalytic cycle, the B12 -binding doare separated by ≈ 50 ˚ main thus must shuttle back and forth between these distant active sites [168]. In the active site of the homocysteine binding domain the substrate forms a metal-ligand cluster with approximately tetrahedral geometry. This result agrees with the measurements showing four sulfur ligands to zinc in homocysteine complexes of E. coli MetH [164] (as mentioned earlier in the section). The crystallographic results on the structure of MetH and the finding of the base-off/His-on binding of the cofactor in a B12 -dependent methyl transferase were consistent with earlier ESR spectroscopic evidence for histidine binding to the cobalt center of p-cresolyl-cobamide (52) in the acetogen Sporomusa ovata [145, 169].

253. 254. 255. 256. 257. 258. 259. A. Butler · B. Kräutler Wetmore SD, Smith DM, Bennett JT, Radom L (2002) J Am Chem Soc 124:14054 Semialjac M, Schwarz H (2003) J Org Chem 68:6967 Semialjac M, Schwarz H (2004) Chem Eur J 10:2781 Tang K-H, Harms A, Frey PA (2002) Biochem 41:8767 Tang K-H, Casarez AD, Wu W, Frey PA (2003) Arch Biochem Biophys 418:49 Chang CH, Frey PA (2000) J Biol Chem 275:106 Berkovitch F, Behshad E, Tang K-H, Enns EA, Frey PA, Drennan CL (2004) Proc Nat Acad Sci USA 101:15870 Blakley RL (1982) In: Dolphin D (ed) B12 , vol II.

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