Description
The tricarboxylic acid (TCA) cycle (Figure 1) is a set of reactions interconnecting nearly all the metabolic pathways occurring within a living cell (Mahler and Cordes, 1966). It catalyzes the oxidation of carbohydrates to CO₂ and furnishes the reducing power necessary to drive the electron transport system, thereby producing adenosine triphosphate (ATP). The TCA cycle also provides a variety of precursors for cellular biosyntheses. Essential for the continuation of the processes is the availability of metabolites capable of generating acetyl coenzyme A and any of the other cycle intermediates. When a hexose or one of its three carbon products is provided, acetyl coenzyme A and oxaloacetate are readily produced for the TCA cycle. For a cell to flourish when a two carbon fragment is the sole available organic nutrient, an alternate pathway is necessary for the production of oxaloacetate. This function can be performed in a variety of microorganisms (Hogg and Kornberg, 1963; Kornberg 1967) including Tetrahymena pyriformis by means of a two-reaction auxiliary to the TCA cycle termed the "glyoxylate bypass" (Hogg, 1959) (Figure 2). The first reaction, catalyzed by isocitrate lyase (E. C. 4.1.3.2) (Wang and Ajl, 1956), is an aldol condensation of glyoxylate with acetyl coenzyme A to form malate. The bypass omits three oxidative steps of the TCA cycle which lead to the loss of two molecules of CO₂. The bypassed cycle effects a net synthesis of a four carbon dicarboxylic acid, succinic acid. from two unit of two carbon fragments in the form of acetyl coenzyme A.
