Alternate Tricarboxylic Acid Cycle

TCA cycle is widely present among all heterotrophs growing aerobically. It operates throughout in the presence of oxygen. However, in some facultative microorganisms, it runs partially with activation of some new enzyme activities and also down-regulation of existing enzymes under anaerobic or microaerophilic conditions. In such conditions, the TCA cycle is branched. Various enzyme activities which are up-regulated or down-regulated are discussed. Further, reductant generation and ATP yields under microaerophilic conditions are also discussed. When Escherichia coli is grown on acetate (2-carbon compound) as a source of carbon, glyoxylate pathway operates, which is a carbon conservation pathway. Details of glyoxylate pathway and its regulation are dealt with. Pathway engineering for over-production of succinic acid is discussed as part of a case study.