title: The catalysis of the 1,1-proton transfer by alpha-methyl-acyl-CoA racemase is coupled to a movement of the fatty acyl moiety over a hydrophobic, methionine-rich surface.
abstract: Alpha-methylacyl-CoA racemases are essential enzymes for branched-chain fatty acid metabolism. Their reaction mechanism and the structural basis of their wide substrate specificity are poorly understood. High-resolution crystal structures of Mycobacterium tuberculosis alpha-methylacyl-CoA racemase (MCR) complexed with substrate molecules show the active site geometry required for catalysis of the interconversion of (2S) and (2R)-methylacyl-CoA. The thioester oxygen atom and the 2-methyl group are in a cis-conformation with respect to each other. The thioester oxygen atom fits into an oxyanion hole and the 2-methyl group points into a hydrophobic pocket. The active site geometry agrees with a 1,1-proton transfer mechanism in which the acid/base-pair residues are His126 and Asp156. The structures of the complexes indicate that the acyl chains of the S-substrate and the R-substrate bind in an S-pocket and an R-pocket, respectively. A unique feature of MCR is a large number of methionine residues in the acyl binding region, located between the S-pocket and the R-pocket. It appears that the (S) to (R) interconversion of the 2-methylacyl chiral center is coupled to a movement of the acyl group over this hydrophobic, methionine-rich surface, when moving from its S-pocket to its R-pocket, whereas the 2-methyl moiety and the CoA group remain fixed in their respective pockets.
authors.affiliation: Biocenter Oulu and Department of Biochemistry, University of Oulu, Linnanmaa, P.O. Box 3000, FIN-90014 University of Oulu, Finland.
authors.forename: Prasenjit
authors.initials: P
authors.lastname: Bhaumik
authors.forename: Werner
authors.initials: W
authors.lastname: Schmitz
authors.forename: Antti
authors.initials: A
authors.lastname: Hassinen
authors.forename: J Kalervo
authors.initials: JK
authors.lastname: Hiltunen
authors.forename: Ernst
authors.initials: E
authors.lastname: Conzelmann
authors.forename: Rik K
authors.initials: RK
authors.lastname: Wierenga
mesh_terms: D000214:Acyl Coenzyme A / Q000737:chemistry / Q000378:metabolism*; D001665:Binding Sites; D002384:Catalysis; D020134:Catalytic Domain; D057927:Hydrophobic and Hydrophilic Interactions; D008715:Methionine / Q000737:chemistry*; D008954:Models, Biological; D008958:Models, Molecular; D008968:Molecular Conformation; D009169:Mycobacterium tuberculosis / Q000201:enzymology; D011522:Protons; D019998:Racemases and Epimerases / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D000214:Acyl Coenzyme A; D011522:Protons; D008715:Methionine; D019998:Racemases and Epimerases