Muscles Krebs cycle

In 1937 Krebs found that citrate could be formed in muscle suspensions if oxaloacetate and either pyruvate or acetate were added. He saw that he now had a cycle, not a simple pathway, and that addition of any of the intermediates could generate all of the others. The existence of a cycle, together with the entry of pyruvate into the cycle in the synthesis of citrate, provided a clear explanation for the accelerating properties of succinate, fumarate, and malate. If all these intermediates led to oxaloacetate, which combined with pyruvate from glycolysis, they could stimulate the oxidation of many substances besides themselves. (Kreb s conceptual leap to a cycle was not his first. Together with medical student Kurt Henseleit, he had already elucidated the details of the urea cycle in 1932.) The complete tricarboxylic acid (Krebs) cycle, as it is now understood, is shown in Figure 20.4. 

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. 

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]. 

Lohman discovered ATP in muscles. Krebs and Henseleit. The urea cycle. Svedberg began studies with the ultracentrifuge. 

The flux through the Krebs cycle can be estimated from the oxygen consumption of a cell or tissue. This has been done in a single human muscle during maximum physical... 

Figure 7.13 Physiological pathway for fatty acid oxidation. The pathway starts with the hormone-sensitive lipase in adipose tissue (the flux-generating step) and ends with the formation of acetyl-CoA in the various tissues. Acetyl-CoA is the substrate for the flux-generating enzyme, citrate synthase, for the Krebs cycle (Chapter 9). Heart, kidney and skeletal muscle are the major tissues for fatty acid oxidation but other tissues also oxidise them.

Figure B2(i) The pathway for conversion of proline and alanine in the flight muscle of the tsetse fly the major ATP-generating pathway. Alanine aminotransferase is essential for the proline oxidation pathway in order for glutamate to enter the Krebs cycle as oxoglutarate and pyruvate to be converted to alanine, the end of the pathway. It is assumed that the pathway is the same for the Colorado beetle, but no studies have been reported.

In any cell that depends on aerobic metabolism, if the rate of ATP utilisation increases, the rate of the Krebs cycle, electron transfer and oxidative phosphorylation must also increase. The mechanism of regulation discussed here is for mammalian skeletal muscle since, to provide sufficient ATP to maintain the maximal power output, at least a 50-fold increase in flux through the cycle is required so that the mechanism is easier to study (Figure 9.22). 

Figure 9.22 The relationship between ATP utilisation by myosin ATPase and ATP generation by Krebs cycle and electron transfer. This relationship between the two major energy systems in muscle is critical. The rate of the cycle and electron transfer is controlled, in part, by ATP utilisation by muscle contraction (see below). This is equivalent to a market economy so that the law of supply and demand applies. The greater the demand and hence the use of ATP, the greater is the rate of generation.

Figure 9.25 Control of the Krebs q/cle and myosin-ATPase by direct effects of Ccf ions and the resultant effects on electron transfer and oxidative phosphorylation in muscle. The stimulation of the Krebs cycle by ions results in an increase in the NADH/NAD concentration ratio, which stimulates electron transfer. The stimulation of myosin-ATPase by Ca lowers the ATP/ADP concentration ratio, which also stimulates electron transfer. The Ca ions are released from the sarcoplasmic reticulum in muscle in response to nervous stimulation. In addition, generation of ADP by myosin ATPase increases the ADP concentration, which stimulates the cycle. Note that a lack of oxygen will prevent generation of ATP (Chapter 13).

A summary of the regulation of the processes of glycoge-nolysis, glycolysis, Krebs cycle, electron transfer and ATP generation is presented in Figure 9.27. Muscle tissue is used as an example but the basis of the mechanism applies to other tissues. 

The oxygen uptake of a maximally working individual muscle in adult humans has been measured in vivo, enabling the calculation of the flux through the Krebs cycle in that muscle to be made (Appendix 9.10). It is compared with the capacity that is calculated from the maximal in vitro activity of oxoglutarate dehydrogenase, in an extract... 

Table 9.6 Flux through Krebs cycle as calculated from the maximum catalytic activity of oxoglutarate dehydrogenase, measured in extracts of muscle, and from oxygen consumption by muscles working maximally...

Table 9.7 Effect of aerobic physical training on the maximum capacity for ATP generation from conversion of glycogen to lactate (glycolysis) and complete oxidation of glucose (the Krebs cycle) in the guadriceps muscle of male and female volunteers...

Box 9.4 Maximum capacity of the Krebs cycle in invertebrate muscles versus that in human muscle... 

Even an invertebrate animal that gives no appearance of physical activity possesses a muscle that has a capacity of the Krebs cycle that is similar to that in a muscle of a young adult human. This is the radular retractor muscle of a mollusc, the wheUc. Whelks are found on the seashore they can use their radula continually for very long periods, up to 24 hours in some cases, to rasp flesh off, for example, a fish carcass. A simple dissection of a whelk readily reveals the radular retractor muscle, easily identified by its brilliant red colour. This muscle illustrates the principle that for muscles that are physiologically essential and have to work for long periods of time, the generation of ATP must be from the oxidation of a fuel which requires mitochondria and therefore cytochromes, which is why the radular retractor muscle is red. 

Pyruvate kinase is a key enzyme in the important process of glycolysis, and its product, pyruvate, is passed into the Krebs cycle. As noted in Section 62.1.2.3.1, pyruvate requires two divalent cations and one monovalent cation for activity. These are usually Mg2+ and K+. The crystal structure of the enzyme from cat muscle has been determined to a resolution of 2.6 A.280... 

Emeretli (1990) demonstrated a convincing link between the activity of succinic dehydrogenase in the red muscle mitochondria of Black Sea species and their motor activity. This enzyme is one of the most important in the Krebs cycle, which controls the intensity of aerobic energy metabolism. 

In most mammalian muscles, amino acids are not a major fuel for oxidative metabolism, although they may be in numerous other animals. When amino acids are the fuels being combusted, they are metabolized by pathways that all ultimately feed into the Krebs cycle, where the intermediates can be fully metabolized (figure 2.3). Being at about the same oxidation state as carbohydrates, the ATP yields of amino acids during oxidation are also similar. For example, alanine oxidation... 

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