Variations of the Acetyl-CoA Pathway in Diversely Related Microorganisms That Are Not Acetogens

Abstract

The capacity of bacteria to totally synthesize acetate from CO2 has been known for approximately 50 years (see Chapter 1, section 1.2.1). However, this process has been unnoticed by most biologists and biochemists. This may be partly due to the fact that essential steps of this process became clear only during the last 15 years (Wood and Ljungdahl, 1991; Ragsdale, 1991; Roberts et al., 1992; see Chapters 1-3 for details). In addition, initially we viewed this process somewhat narrowly relative to the organisms that make use of it. However, this pathway and variations thereof are used by diversely related microorganisms which have in common the key catalyst of the pathway, the nickel-containing enzyme CO dehydrogenase/acetyl-CoA synthase. It catalyzes not only CO oxidation to CO2 according to the reaction CO + H2O → CO2 + 2[H], but also CO2 reduction to an enzyme-bound carbonyl [CO] via the reaction CO2 + 2[H] → enzyme-[CO] + H2O. The carbonyl-level carbon is then combined with a tetrahydrofolate-derived methyl-level carbon (via an enzyme-bound [CH3]) and coenzyme A (CoA) to give acetyl-CoA. The variants of this biochemical principle are widely distributed in anaerobic Eubacteria and Archaebacteria and fulfill different central metabolic functions. Many different aspects of this pathway, termed the acetyl-CoA “Wood” pathway, have been covered in preceding chapters [see also Fuchs (1986, 1989); Ljungdahl (1986); Ragsdale (1991); Thauer (1988, 1989, 1990); Thauer et al. (1989); Wood and Ljungdahl (1991); Wood et al. (1986a, 1986b, 1986c); Diekert (1992); Drake (1992,1993)]. In this chapter, five aspects of the acetyl-CoA pathway and its variants (Fig. 19.1) which are of general biological relevance are briefly discussed.