7-Mercaptoheptanoylthreonine phosphate

Abbreviations MFR, methanofuran H4MPT, tetrahydromethanoptcrin H-S-CoM, coenzyme M H-S-HTP, A7-7-mercaptoheptanoylthreonin phosphate. 

The final step in methanogenesis is the reductive demethylation of CH3-S-CoM to CH4. This reduction involves two reactions CH3-S-C0M is reduced with jV-7-mercaptoheptanoylthreonine phosphate (H-S-HTP) (Fig. 2B) as electron donor to yield CH4 and a heterodisulfide of H-S-CoM and H-S-HTP (CoM-S-S-HTP) (Reaction 7, Table 2). This reaction is catalyzed by CH3-S-C0M reductase [70-72] which contains a nickel porphinoid, factorF430, as prosthetic group (Fig. 2C) (for a recent review see Friedmann et al. [73]). The subsequent reduction of the heterodisulfide with H2 to yield H-S-HTP and H-S—CoM (Reaction 8, Table 2) is catalyzed by CoM-S-S-HTP-dependent heterodisulfide reductase. The enzyme is an iron-sulfur protein containing FAD as prosthetic group [74]. The physiological electron donor for the heterodisulfide reductase is not known. 

Fig. 5. Proposed mechanism of ATP synthesis coupled to methyl-coenzyme M (CH3-S-C0M) reduction to CH4 The reduction of the heterodisulfide (CoM-S-S-HTP) as a site for primary translocation. ATP is synthesized via membrane-bound -translocating ATP synthase. CoM-S-S-HTP, heterodisulfide of coenzyme M (H-S-CoM) and 7-mercaptoheptanoylthreonine phosphate (H-S-HTP) numbers in circles, membrane-associated enzymes (1) CH3-S-C0M reductase (2) dehydrogenase (3) heterodisulfide reductase 2[H] can be either H2, reduced coenzymeF420 F420H2) or carbon monoxide the hatched box indicates an electron transport chain catalyzing primary translocation the stoichiometry of translocation (2H /2e , determined in everted vesicles) was taken from ref. [117] z is the unknown If /ATP stoichiometry A/iH, transmembrane electrochemical...

Nell KM, Rinehart KL Jr, Tanner RS, Wolfe RS (1986) Structure of component B (7-mercaptoheptanoylthreonine phosphate) of the methylcoenzyme M methylreductase system of Methanobacterium thermoautotrophicum. Proc Natl Acad Sci USA 83 4238-4242... 

Oxoglutarate can also serve as a starter piece for elongation by the oxoacid pathway. Extension by three carbon atoms yields 2-oxosuberate (Eq. 21-1). This dicarboxylate is converted by reactions shown in Eq. 24-39 into biotin and in archaebacteria into the coenzyme 7-mercaptoheptanoylthreonine phosphate (HTP), Eq. 21-1. Lipoic acid is also synthesized from a fatty acid, the eight-carbon octanoate. A fatty acid synthase system that utilizes a mitochondrial ACP may have as its primary fimction the synthesis of ocfanoate for lipoic acid formation. The mechanism of insertion of the two sulfur atoms to form lipoate (Chapter 15) is imcerfain. If requires an iron-sulfur protein jg probably similar to the corresponding process in the synthesis of biotin (Eq. 24-39)9 93a formation of HTP (Eq. 21-1). One component of the archaebacterial cofactor methano-furan (Chapter 15) is a tetracarboxylic acid that is formed from 2-oxoglufarafe by successive condensations with two malonic acid imits as in fatty acid synthesis. ... 

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