The Krebs cycle

The Krebs cycle is sometimes still referred to as the citric acid cycle, citric acid being one of the intermediates involved, and even the tricarboxylic acid cycie, in that several of the intermediates are tri-acids. As the name suggests, the process is a cycle, so that there is a reasonably constant pool of intermediates functioning in an organism, and material for degradation is processed via this pool of intermediates. Overall, though, the material processed does not increase the size of the pool. The compound 

We have just seen that anaerobic organisms metabolize pyruvate from the glycolytic pathway by various means, but that the prime objective is to reoxidize NADH to NAD+. In aerobic organisms, reoxidation of NADH is achieved via oxidative phosphorylation, generating ATP in the process, and 

The whole process is multi-step, and catalysed by the pyruvate dehydrogenase enzyme complex, which has three separate enzyme activities. Dnring the transformation, an acetyl group is effectively removed from pyruvate, and passed via carriers thiamine 

The requirement for NAD+ is to reoxidize the lipoic acid carrier. It is worth mentioning that the pyruvate acetaldehyde conversion we considered at the end of the glycolytic pathway involves the same initial sequence, and pyruvate decarboxylase is another thiamine diphosphate-dependent enzyme. 

Acetyl-CoA (see Box 10.8) is a thioester of acetic acid with coenzyme A. It is a remarkably common intermediate in many metabolic degradative and synthetic pathways, for which the reactivity of the thioester fnnction plays a critical role. There are two major sonrces of the acetyl-CoA entering the Krebs cycle glycolysis via the oxidative 

The Krebs cycle is a series of reactions catalyzed by sev en enzymes in mitochondria. Its function is to catalyze removal of electrons from nutrients and to transfer them to NAD and FAD, producing NADH plus H, and FADHj, respectively. These reduced cofactors exist only momentarily in their reduced (err oxidized) forms as they continually accept and then donate electrons to the respiratory chain. The respiratory chain, composed of a number of cytochromes, uses electrons for reduction of to water, This reduction process is accompanied by or coupled with the regeneration of ATP, that is, conversion of A DP back to ATE The overall effect may be summarized thus The Krebs cycle and respiratory chain arc used for oxidizing nutrients io COy and for the production of energy. 

The Krebs cycle, which is also called the citric add cycle or the tricarboxylic acid cycle, was discovered by Hans Krebs in 1 )37, at Cambridge University in Great Britain (Holmes, 1993). 

Carbohydrates are introduced into the Krebs cycle at the point where pyruvate dehydrogenase catalyzes conversion of pyruvate to acetyl-Co A with the concomitant reduction of NAD. Citrate synthase catalyzes introduction of the 2-carbon unit of acetyl-CoA into the Krebs cycle. Pyruvate can arise from glucose, fructose, lactate, alanine, and glycerol. Acetyl-CcjAcan arise from pyruvate, as well as from fatty acids. Oxidation of fatty acids results in production of acetyl-Co A, which enters the Krebs cycle at the point catalyzed by citrate synthase. Breakdown of ketogenic amino adds also results in the production of acctyl-CoA, which enters the Krebs cycle at this point. Citrate and malate occur in high cor centrations in certain fruits and vegetables. These chemicals directly enter the Krebs cycle at the indicated points. 

A number of amino acids can be broken down to produce ct-kctoglutarate, succinyl-CoA, or oxaJoacetic acid, intermediates of the Krebs cycle. In this process, the carbon skeletons of the amino adds are broken down to form intermediates of the Krebs cycle. The glucogenic amino acids also can be metabolized in this way. 

FtCURE 4.58 The Krebs cycle — names of Irttermediates. The intermediates in the cycle are citrate, isocitrate, a-ketoglutarate, succiny -CoA, succinate, fumarate, mala re, and oxaloacetate. Acetyl-CoA i.s used to introduce the acetyl group to the Krebs cycle. Carbon dioxide is its final product. 

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Glutamic acid is formed m most organisms from ammonia and a ketoglutaric acid a Ketoglutaric acid is one of the intermediates m the tricarboxylic acid cycle (also called the Krebs cycle) and arises via metabolic breakdown of food sources carbohy drates fats and proteins... 

The August 1986 issue of the Journal of Chemical Educa tion (pp 673-677) contains a review of the Krebs cycle... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Physiological Role of Citric Acid. Citric acid occurs ia the terminal oxidative metabolic system of virtually all organisms. This oxidative metabohc system (Fig. 2), variously called the Krebs cycle (for its discoverer, H. A. Krebs), the tricarboxyUc acid cycle, or the citric acid cycle, is a metaboHc cycle involving the conversion of carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organism s growth, movement, luminescence, chemosynthesis, and reproduction. The cycle also provides the carbon-containing materials from which cells synthesize amino acids and fats. Many yeasts, molds, and bacteria conduct the citric acid cycle, and can be selected for thek abiUty to maximize citric acid production in the process. This is the basis for the efficient commercial fermentation processes used today to produce citric acid. 

Glyoxylate cycle A modification of the Krebs cycle, which occurs in some bacteria. Acetyl coenzyme A is generated directly from oxidation of fatty acids or other lipid compounds. 

The citric acid cycle, a nine-step process, also diverts chemical energy to the production of ATP and the reduction of NAD and FAD. In each step of the citric acid cycle (also known as the Krebs cycle) a glucose metabolite is oxidized while one of the carrier molecules, NAD or FAD, is reduced. Enzymes, nature s chemical catalysts, do a remarkable job of coupling the oxidation and reduction reactions so that energy is transferred with great efficiency. 

These points have important functional implications. While neuronal glutamate may come from glucose via pyruvate, the Krebs cycle and transamination of alpha-oxoglutamate, it seems likely that most of the transmitter originates from the deamination of glutamine. After release, the high-affinity uptake sites (transporters)... 

NADH, which enters the Krebs cycle. However, during cerebral ischaemia, metabolism becomes anaerobic, which results in a precipitous decrease in tissue pH to below 6.2 (Smith etal., 1986 Vonhanweh etal., 1986). Tissue acidosis can now promote iron-catalysed free-radical reactions via the decompartmentalization of protein-bound iron (Rehncrona etal., 1989). Superoxide anion radical also has the ability to increase the low molecular weight iron pool by releasing iron from ferritin reductively (Thomas etal., 1985). Low molecular weight iron species have been detected in the brain in response to cardiac arrest. The increase in iron coincided with an increase in malondialdehyde (MDA) and conjugated dienes during the recirculation period (Krause et al., 1985 Nayini et al., 1985). 

The oxidation products of an even-numbered fatty acid are acetyl-CoA, FAD H2 and NAD H2. Subsequently, acetyl-CoA enters the Krebs cycle, and FAD H2 and NAD H2 are directly supplied to the respiratory chain. 

Carboxylation of propionyl-CoA is accomplished by propionyl-CoA carboxylase (biotin, which is the carboxyl group carrier, serves as a coenzyme for this enzyme) the presence of ATP is also required. The methylmalonyl-CoA formed is converted by methylmalonyl-CoA mutase (whose coenzyme, deoxyadenosylcobalamin, is a derivative of vitamin B]2) to succinyl-CoA the latter enters the Krebs cycle. 

In the liver, the ketone bodies suffer no transformation, and are excreted into the blood. The normal contents of ketone bodies (as acetoacetate or P-hydroxy-butyrate) amount to mere 0.1-0.6 mmol/ litre). Other tissues and organs (heart, lung, kidney, muscle, and nervous tissue), as distinct from the liver, utilize the ketone bodies as energy substrates. In the cells of these tissues, acetoacetate and 1-hydroxybutyrate enter ultimately the Krebs cycle and burn down to C02 and H,0 to release energy. 

Wachtcrshauser s prime candidate for a carbon-fixing process driven by pyrite formation is the reductive citrate cycle (RCC) mentioned above. Expressed simply, the RCC is the reversal of the normal Krebs cycle (tricarboxylic acid cycle TCA cycle), which is referred to as the turntable of metabolism because of its vital importance for metabolism in living cells. The Krebs cycle, in simplified form, can be summarized as follows ... 

Since, in order to decarboxylate pyruvate, the cofactor (6-thioctic acid) must be in its oxidized form, Calvin suggested that, in the presence of light, the coenzyme shifts to the reduced (dithiol) form, thus markedly reducing the rate of incorporation of C14 into the Krebs cycle. Furthermore, Bradley and Calvin236(f) suggested that 6-thioctic acid is an acceptor of... 

The tricarboxylic acid cycle is also known as the Krebs cycle or the citric acid cycle. Why give something so central to life only one name ... 

The biochemical classification of mitochondrial DNA is based on the five major steps of mitochondrial metabolism. These steps are illustrated in Figure 42-3 and divide mitochondrial diseases into five groups defects of mitochondrial transport, defects of substrate utilization, defects of the Krebs cycle, defects of the respiratory chain and defects of oxidation-phosphorylation coupling. 

Defects of the Krebs cycle. Fumarase deficiency was reported in children with mitochondrial encephalomyop-athy. Usually, there is developmental delay since early infancy, microcephaly, hypotonia and cerebral atrophy, with death in infancy or early childhood. The laboratory hallmark of the disease is the excretion of large amounts of fumaric acid and, to a lesser extent, succinic acid in the urine. The enzyme defect has been found in muscle, liver and cultured skin fibroblasts [16]. 

De Meirleir,L. Defects of pyruvate metabolism and the Krebs cycle. /. Child Neurol. 17(Suppl. 3) S26-S33,2002. 

Biodegradation of the aliphatic polyesters occurs by bulk erosion. The lactide/gly-colide polymer chains are cleaved by random nonenzymatic hydrolysis to the monomeric lactic and glycolic acids and are eliminated from the body through the Krebs cycle, primarily as carbon dioxide and in urine. 

The global outcome of the Krebs cycle is that one molecule of acetyl CoA is converted to three molecules of NADH, two of C02, one of GTP and one of FADH2. The reducing equivalents will be used, as we will see later, to generate ATP. 

We will now draw attention to the Krebs cycle otherwise called the tricarboxylic acid cycle (fig. 17). It is now known that carbohydrate metabolism and fatty acid metabolism as well as acetate proceed via changes indicated in the cycle. The essential... 

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