Citrate metabolism in anaerobic bacteria

Abstract

The regulation of anaerobic citrate metabolism is very diverse among different groups of bacteria. In organisms like Streptococcus lactis and Clostridium sporosphaeroides which lack citrate synthase, the activity of its antagonistic enzyme, citrate lyase, need not be regulated. Many anaerobes like Rhodocyclus gelatinosus and Clostridium sphenoides are able to synthesize their own l‐glutamate and contain citrate synthase. In these bacteria the activity of citrate metabolizing enzymes which are involved in a cascade system are under strict control. In Rc. gelatinosus activation/inactivation of citrate lyase is controlled by acetylation/deacetylation which is catalyzed by its corresponding regulatory enzymes, citrate lyase ligase and citrate lyase deacetylase. In C. sphenoides inactivation of citrate lyase is accomplished by deacetylation as well as by changing in the enzyme conformation. Activation of citrate lyase is catalyzed by citrate lyase ligase whose activity in addition is modulated by phosphorylation/dephosphorylation. Further, electron transport process also seems to play a role in the inactivation of citrate metabolizing enzymes in enteric bacteria.

Anaerobic degradation of citrate under sulfate reducing and methanogenic conditions

“…Under anaerobic conditions, bacteria cannot fully oxidize citrate with the TCA cycle due to the limited possibilities of recycling reduced electron carriers. In order to degrade citric acid without elemental oxygen, bacteria have developed unique fermentation pathways such as the production of pyruvate, acetate and carbon dioxide, referred to as the ''citrate fermentation'' pathway (Antranikian and Giffhorn 1987;Antranikian and Gottschalk 1989;Bott et al 1995). The pathway is initiated by citrate lyase that converts citrate to oxaloacetate and acetate.…”

“…Citrate is mainly used in the food industry and it was originally synthesized from citrus fruits, which have high contents of this compound (Antranikian and Giffhorn 1987). Citrate is also a naturally occurring chelating agent that forms soluble multidentate complexes with metals.…”

“…Citrate can potentially serve as an electron donor for sulfate reduction applied to promote the removal of metals (Foucher et al 2001;Hulshoff Pol et al 2001;Quan et al 2003;Tabak et al 2003), and it can also potentially be used by methanogens that coexist in anaerobic biofilms. Complete degradation of citrate to CO 2 by aerobic bacteria is known to proceed via the tricarboxylic acid (TCA) cycle (Antranikian and Giffhorn 1987). Under anaerobic conditions, bacteria cannot fully oxidize citrate with the TCA cycle due to the limited possibilities of recycling reduced electron carriers.…”

Influence of metal complexation on the metabolism of citrate byKlebsiella oxytoca

“…Because of its ubiquitous distribution, citrate can easily be degraded by a large variety of bacterial species via the tricarboxylic acid cycle or the citrate fermentation pathway (Antranikian & Giffhorn 1987). The tricarboxylic acid cycle is the expected pathway for citrate degradation under the aerobic conditions of this investigation.…”

Using a carbon-based ASM3 EAWAG Bio-P for modelling the enhanced biological phosphorus removal in anaerobic/aerobic activated sludge systems

“…On the other hand, citrate is abundant in nature, and found in every cell. Further, most anaerobes have evolved a ''citrate fermentation pathway'', in which pyruvate, acetate and carbon dioxide are formed by citrate lyase and oxalacetate decarboxylase according to the following equations [24]:…”