PDBsum entry 1m1m

Transferase

PDB id: 1m1m

Contents

Protein chains

332 a.a. *

Waters ×78

* Residue conservation analysis

References listed in PDB file

Key reference

Title

Probing the mechanism of the mycobacterium tuberculosis beta-Ketoacyl-Acyl carrier protein synthase III mtfabh: factors influencing catalysis and substrate specificity.

Authors

A.K.Brown, S.Sridharan, L.Kremer, S.Lindenberg, L.G.Dover, J.C.Sacchettini, G.S.Besra.

Ref.

J Biol Chem, 2005, 280, 32539-32547. [DOI no: 10.1074/jbc.M413216200]

PubMed id

16040614

Abstract

Mycolic acids are the dominant feature of the Mycobacterium tuberculosis cell wall. These alpha-alkyl, beta-hydroxy fatty acids are formed by the condensation of two fatty acids, a long meromycolic acid and a shorter C(24)-C(26) fatty acid. The component fatty acids are produced via a combination of type I and II fatty acid synthases (FAS) with FAS-I products being elongated by FAS-II toward meromycolic acids. The beta-ketoacyl-acyl carrier protein (ACP) synthase III encoded by mtfabH (mtFabH) links FAS-I and FAS-II, catalyzing the condensation of FAS-I-derived acyl-CoAs with malonyl-acyl carrier protein (ACP). The acyl-CoA chain length specificity of mtFabH was assessed in vitro; the enzyme extended longer, physiologically relevant acyl-CoA primers when paired with AcpM, its natural partner, than with Escherichia coli ACP. The ability of the enzyme to use E. coli ACP suggests that a similar mode of binding is likely with both ACPs, yet it is clear that unique factors inherent to AcpM modulate the substrate specificity of mtFabH. Mutation of proposed key mtFabH residues was used to define their catalytic roles. Substitution of supposed acyl-CoA binding residues reduced transacylation, with double substitutions totally abrogating activity. Mutation of Arg(46) revealed its more critical role in malonyl-AcpM decarboxylation than in the acyl-CoA binding role. Interestingly, this effect was suppressed intragenically by Arg(161) --> Ala substitution. Our structural studies suggested that His(258), previously implicated in malonyl-ACP decarboxylation, also acts as an anchor point for a network of water molecules that we propose promotes deprotonation and transacylation of Cys(122).

Figure 5.
FIGURE 5. Conserved water molecules near the active site residues in mtFabH structures. Arg46-Arg161 Ala mutant (blue) and 1HZP (magenta) structures are superposed. Residues and water (wat) molecules are numbered as in the Arg46-Arg161 Ala mutant structure. The figure was made using Xtalview (37).

Figure 6.
FIGURE 6. Proposed mechanism for mtFabH transacylation. The formation of the thiolate ion at Cys122, which is crucial to the transacylation reaction, appears to be promoted in part by the helix dipole effect (represented here by a partial positive charge at the N-terminal end of helix 5) and by shuttling of the proton via water 568 and ultimately abstraction via N 2 of His258. Through hydrogen bonding, the backbone nitrogens of Gly322 and Cys122 stabilize the negative charge gained by the acyl-CoA carbonyl during formation of the acyl-enzyme thioester Michaelis complex (boxed). Data are adapted from Refs. 24 and 29.

The above figures are reprinted by permission from the ASBMB: J Biol Chem (2005, 280, 32539-32547) copyright 2005.