Regulation of the TCA cycle

TCA cycle substrates oxaloacetate and acetyl-CoA and the product NADH are the critical regulators. The availability of acetyl-CoA is regulated by pyruvate dehydrogenase complex. The TCA cycle enzymes citrate synthase. 

Availability of acetyl CoA from pyruvate is controlled by PDH activity, which is regulated by the concentration of NADH and the ADP/ATP ratio. 

Isocitrate dehydrogenase is allosterically inhibited by NADH, an indicator of the availability of high levels of energy. 

The enzyme is activated by ADP and Ca, which signal a need for energy in the cell. 

Conversion of a-ketoglutarate to succinyl CoA, catalyzed by a-ketoglutarate dehydrogenase, is inhibited by NADH and ATP. 

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When the carbohydrates are being metabolized, TCA ycle intermediates are replenished by production of oxalo-etate from pyruvate. In mammals, this reaction is cata-[zed by pyruvate carboxylase, and one ATP-to-ADP concision is associated with the carboxylation. Other operties of this reaction are discussed later in this chapter l connection with regulation of the TCA cycle and related [etabolic sequences. 

In order to understand the regulation of the TCA cycle, it is necessary to look at the AG values for the various reactions and the kinetic properties of the enzymes. Values for the non-equilibrium reactions are tabulated below as Table 9.4 ... 

Regulation of the TCA cycle serves two functions it ensures that NADH is generated fast enough to maintain ATP homeostasis and it regulates the concentration of TCA cycle intermediates. For example, in the fiver, a decreased rate of isocitrate dehydrogenase increases citrate concentration, which stimulates citrate efflux to the cytosol. A number of regulatory interactions occur in the TCA cycle, in addition to those mentioned above, that control the levels of TCA intermediates and their flux into pathways that adjoin the TCA cycle. 

Walshaw D.L., Wilkinson A., Mundy M., Smith M. and Poole P.S., 1997, Regulation of the TCA cycle and the general aminoacid permease by overflow metabolism in Rhizobium leguminosarum. Microbiology 143 2209-2221. 

Citrate synthase is the first step in this metabolic pathway, and as stated the reaction has a large negative AG°. As might be expected, it is a highly regulated enzyme. NADH, a product of the TCA cycle, is an allosteric inhibitor of citrate synthase, as is succinyl-CoA, the product of the fifth step in the cycle (and an acetyl-CoA analog). 

At least two enzymes compete for acetyl-CoA - the citrate synthase and 3-ke-tothiolase. The affinities of these enzymes differ for acetyl-CoA (Table l),and at low concentrations of it the citrate synthase reaction tends to dominate, provided that the concentration of 2/H/ is not inhibiting. The fine regulation of the citrate synthases of various poly(3HB) accumulating bacteria has been studied [ 14, 47, 48]. They appear to be controlled by cellular energy status indicators (ATP, NADH, NADPH) and/or intermediates of the TCA cycle. The 3-ketothio-lase has also been investigated [10-14,49, 50]. This enzyme is, above all, inhibited by CoASH [10,14,49]. This important feature will be further considered below. 

K2. Katunuma, N., Okada, M., and Nishi, Y., Regulation of the urea cycle and TCA cycle by ammonia. Advan. Enzyme Regul. 4, 317-335 (1966). 

In the cell, compartmentation of enzymes into multienzyme complexes or organelles provides a means of regulation, either because the compartment provides unique conditions or because it limits or channels access of the enzymes to substrates. Enzymes or pathways with a common function are often assembled into organelles. For example, enzymes of the TCA cycle are all located within the mitochondrion. The enzymes catalyze sequential reactions, and the product of one reaction is the substrate for the next reaction. The concentration of the pathway intermediates remains much higher within the mitochondrion than in the surrounding cellular cytoplasm. 

The TCA cycle occurs in the mitochondrion, where its flux is tightly coordinated with the rate of the electron transport chain and oxidative phosphorylation through feedback regulation that reflects the demand for ATP. The rate of the TCA cycle is increased when ATP utilization in the cell is increased through the response of several enzymes to ADP levels, the NADH/ NAD ratio, the rate of FAD(2H) oxidation or the Ccf concentration. For example, isocitrate dehydrogenase is allosterically activated by ADP. 

The oxidation of acetyl CoA in the TCA cycle and the conservation of this energy as NADH and FAD(2H) is essential for generation of ATP in almost all tissues in the body. In spite of changes in the supply of fuels, type of fuels in the blood, or rate of ATP utihzation, cells maintain ATP homeostasis (a constant level of ATP). The rate of the TCA cycle, like that of all fuel oxidation pathways, is principally regulated to correspond to the rate of the electron transport chain, which is regulated by the ATP/ADP ratio and the rate of ATP utilization (see Chapter 21). The major sites of regulation are shown in Fig 20.12. 

Compartmentation plays an important role in regulation. The close association between the rate of the electron transport chain and the rate of the TCA cycle is maintained by their mutual access to the same pool of NADH and NAD in the mitochondrial matrix. NAD, NADH, CoASH, and acyl CoA derivatives have no transport proteins and cannot cross the mitochondrial membrane. Thus, all of the dehydrogenases compete for the same NAD molecules, and are inhibited when NADH rises. Likewise, accumulation of acyl CoA derivatives (e.g., acetyl CoA) within the mitochondrial matrix affects other CoA-utilizing reactions, either by competing at the active site or limiting CoASH availability. 

Ozaki, H. and Shiio, I. (1969) Regulation of the TCA and glyoxylate cycles in Brevibacterium flavum. 

Ozaki, H. and Shiio, I. (1968) Regulation of the TCA and glyoxylate cycles in Brevibacterium jlavum. I. Inhibition of isocitrate lyase and isocitrate dehydrogenase by organic acids related to the TCA and glyoxylate cycles. 

Malic enzymels), L-malatt-NADP oxidortdm-tase, decarboxylating (EC 1.1.1.40) an important enzyme found in most organisms, which catalyses the decarboxylation of L-malate to pyruvate and CO2, with concomitant reduction of NADP to NADPH (or the synthesis of malate by the reverse reaction) HOOC-CHj-CHOH-COOH + NADF CHj-CO-COOH + CO2 + NADPH + H. M. e. has various metabolic roles 1. Synthesis of malate by the action of M.e. may serve as an Anaplerotic reaction (see) of the TCA-cycle 2. An important route for the total combustion of any TCA-[Pg.380]

As an intermediate of the TCA cycle 2-oxoglutaiic acid (2-OGA) - as well as CA -can be exocellularly exuded by microorganisms (Table 19.2). The intracellular energy regulation under conditions of growth limitation and unlimited substrate uptake of the cells grown are important. A precondition for this process is the limitation of reproductive cell growth by nutrient exhaustion from the culture broth. In the case of 2-OGA, thiamine limitation is also necessary because of the thiamine... 

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