Menaquinone (vitamin K2
) is an important cofactor that is exploited in electron-transport pathways (Meganathan, 2001
). The vitamin consists of a naphthoquinone moiety with a polyisoprenyl substituent, the length of which varies in different bacteria. Humans lack the enzymes that synthesize this vitamin and acquire it from diet or from intestinal bacteria and this absence contributes to the potential value of these enzymes as therapeutic targets, especially for important pathogenic bacteria such as Mycobacterium tuberculosis
and Staphylococcus aureus
The biosynthesis of menaquinone has been studied extensively in Escherichia coli
(Bentley & Meganathan, 1982
; Lin & Kuritzkes, 1987
; Meganathan, 1996
) and also in Bacillus subtilis
and M. pheli
(Rowland et al.
). The biosynthesis typically involves six to eight enzymes and in a number of cases the genes encoding these enzymes have been proven to be essential to the bacteria by genetic methods. The structures of three of the biosynthetic enzymes have been characterized: MenC and MenF from E. coli
(Palmer et al.
; Thompson et al.
; Kolappan et al.
) and MenB from M. tuberculosis
(Truglio et al.
; Johnston et al.
Our interest is in MenB, 1,4-dihydroxy-2-naphthoate synthase (EC 188.8.131.52). This enzyme converts O
-succinylbenzoyl-CoA (the CoA ester of O
-succinylbenzoic acid; OSB-CoA) to 1,4-dihydroxy-2-naphthoyl-CoA (the CoA ester of 1,4-dihydroxy-2-naphthoic acid; Fig. 1
). The menB
gene is essential in S. aureus
(Forsyth et al.
), B. subtilis
(Kobayashi et al.
) and Haemophilus influenzae
(Akerley et al.
(a) The reaction catalyzed by MenB. (b) Chemical structure of the ligand acetoacetyl-CoA.
The crystal structure of MenB from M. tuberculosis
MenB) has been reported in the apo form and in complex with acetoacetyl-CoA or napthyl-CoA (Truglio et al.
; Johnston et al.
). These early studies confirmed MenB to be a member of the crotonase superfamily of enzymes, most of which are functional trimers or hexamers. A common feature of this group is that the substrates are CoA derivatives and the mechanism involves the stabilization of a thioester enolate by an oxyanion hole (Xiang et al.
; Gerlt & Babbitt, 2001
Seeking to provide a template to support structure-based inhibitor development, we initiated a study of S. aureus 1,4-dihydroxy-2-naphthoate synthase (SaMenB). The substrate OSB-CoA is unstable; therefore, in order to obtain details of molecular interactions in the active site we studied the complex with acetoacetyl-CoA (Fig. 1