Crystal Structure of Escherichia coli Crotonobetainyl-CoA: Carnitine CoA-Transferase (CaiB) and Its Complexes with CoA and Carnitinyl-CoA †

Department of Biochemistry, McGill University, Montréal, Quebec, Canada
Biochemistry (Impact Factor: 3.02). 05/2005; 44(15):5728-38. DOI: 10.1021/bi047656f
Source: PubMed


L-Carnitine (R-[-]-3-hydroxy-4-trimethylaminobutyrate) is found in both eukaryotic and prokaryotic cells and participates in diverse processes including long-chain fatty-acid transport and osmoprotection. The enzyme crotonobetainyl/gamma-butyrobetainyl-CoA:carnitine CoA-transferase (CaiB; E.C. 2.8.3.-) catalyzes the first step in carnitine metabolism, leading to the final product gamma-butyrobetaine. The crystal structures of Escherichia coli apo-CaiB, as well as its Asp169Ala mutant bound to CoA and to carnitinyl-CoA, have been determined and refined to 1.6, 2.4, and 2.4 A resolution, respectively. CaiB is composed of two identical circular chains that together form an intertwined dimer. Each monomer consists of a large domain, containing a Rossmann fold, and a small domain. The monomer and dimer resemble those of formyl-CoA transferase from Oxalobacter formigenes, as well as E. coli YfdW, a putative type-III CoA transferase of unknown function. The CoA cofactor-binding site is formed at the interface of the large domain of one monomer and the small domain from the second monomer. Most of the protein-CoA interactions are formed with the Rossmann fold domain. While the location of cofactor binding is similar in the three proteins, the specific CoA-protein interactions vary somewhat between CaiB, formyl-CoA transferase, and YfdW. CoA binding results in a change in the relative positions of the large and small domains compared with apo-CaiB. The observed carnitinyl-CoA product in crystals of the CaiB Asp169Ala mutant cocrystallized with crotonoyl-CoA and carnitine could result from (i) a catalytic mechanism involving a ternary enzyme-substrate complex, independent of a covalent anhydride intermediate with Asp169, (ii) a spontaneous reaction of the substrates in solution, followed by binding to the enzyme, or (iii) an involvement of another residue substituting functionally for Asp169, such as Glu23.

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    • "Rather, its closest homologue with known function is CaiB of E. coli, a Class III acyl CoA transferase that donates acetyl CoA to carnitine when that amino acid is used as a terminal electron acceptor in anaerobic conditions. CaiB is a homo-dimer (Rangarajan et al., 2005), but DddD is ; twice the size of CaiB, and has a tandemly repeated domain structure that likely forms an 'intramolecular dimer'. Nevertheless, by analogy with the function of CaiB, it is predicted that DddD adds an acyl CoA moiety to DMSP, prior to its subsequent cleavage and release of DMS (Todd et al., 2007; Fig. 1). "
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