The Catabolism of Carbohydrates

To complete the oxidation of fatty acids the acetyl units of acetyl-CoA generated in the P oxidation sequence must be oxidized to carbon dioxide and water.77 The citric acid (or tricarboxylic acid) cycle by which this oxidation is accomplished is a vital part of the metabolism of almost all aerobic creatures. It occupies a central position in metabolism because of the fact that acetyl-CoA is also an intermediate in the catabolism of carbohydrates and of many amino acids and other compounds. The cycle is depicted in detail in Fig. 10-6 and in an abbreviated form, but with more context, in Fig. 17-4. 

The interconversion of the two triosephosphates is an essential step in the catabolism of carbohydrates (see chapter 12). Examining the catalytic mechanism of triosephosphate isomerase is instructive. It is among the enzymes that ap-... 

Major pathways of purine degradation in animals. Primates excrete uric acid. Mammals other than primates catabolize uric acid to other end products. In contrast to the catabolism of carbohydrates, lipids, or... 

The citric acid cycle is a sequence of reactions in which the two carbon atoms of acetyl-CoA are ultimately oxidized to C02. It is the central pathway for the release of energy from acetyl-CoA, which is produced from the catabolism of carbohydrates (Chap. 11), fatty acids (Chap. 13), and some amino acids (Chap. 15) and is closely involved with two other processes, namely, electron transport and oxidative phosphorylation (Chap. 14). 

The goal of glycolysis, glycogenolysis and the TCA cycle, is to conserve energy as ATP from the catabolism of carbohydrates. If cells have sufficient supplies of ATP then these pathways and cycles are inhibited under such conditions the liver will convert a variety of excess molecules into glucose and/or glycogen. 

Cellular metabolism results in the production of large quantities of hydrogen that need to be excreted in order to maintain acid-base balance. In addition, small amounts of acid and alkali are also presented to the body through the diet. The bulk of acid production is in the form of CO2, from the metabolism of carbohydrates, proteins, and lipids. When respiratory function is normal, the amount of CO2 produced metabolically is equal to the amount lost by respiration, and the blood CO2 concentration remains constant. The average adult produces approximately 15,000 mmol of CO2 each day from the catabolism of carbohydrate, protein, and fat. ... 

Most of tbe energy that the body requires for maintenance, work, and growth is obtained by the terminal oxidation of acetyl coenzyme A (acetyl-CoA), which is produced by the catabolism of carbohydrates, fatty acids, and amino acids. 

The rates of the oxidative steps in the citric acid cycle are limited by the rate of reoxidation of NADH and reduced ubiquinone in the electron transport chain which may sometimes be restricted by the availability of O2. However, in aerobic organisms this rate is usually determined by the concentration of ADP and/or P available for conversion to ATP in the oxidative phosphorylation process (Chapter 18). If catabolism supplies an excess of ATP over that needed to meet the cell s energy needs, the concentration of ADP falls to a low level, cutting off phosphorylation. At the same time, ATP is present in high concentration and acts as a feedback inhibitor for the catabolism of carbohydrates and fats. This inhibition is exerted at many points, a few of which are indicated in Fig. 

FIGURE 19.10 a summary of catabolism, showing the central role of the citric acid cycle. Note that the end products of the catabolism of carbohydrates, lipids, and amino acids all appear. (PEP is phosphoenolpyruvate a-KG is a-ketoglutarate TA is transamination > > > is a multistep pathway.)... 

Acetyl-GoA is the starting point for lipid anabolism in both plants and animals. An important source of acetyl-GoA is the catabolism of carbohydrates. We have just seen that animals cannot convert lipids to carbohydrates, but they can convert carbohydrates to lipids. The efficiency of the conversion of carbohydrates to lipids in animals is a source of considerable chagrin to many humans (see the Biochemical Connections box on page 572). 

Glycolysis The initial pathway in the catabolism of carbohydrates, by which a molecule of glucose is broken down to two molecules of pyruvate, with a net production of ATP molecules and the reduction of two NAD molecules to NADH. Under aerobic conditions, these NADH molecules are reoxidized by the electron transport chain under anaerobic conditions, a different electron acceptor is used. An anaerobic metabolic pathway used to break down glucose into pyruvic acid while producing some ATP. 

In addition to the role of niacin in the degradation of proteins, niacin has a very important role in the catabolism of carbohydrates. We have described the evidence that niacin plays a crucial role in the conversion of glucose to the energy substrate ATP (Bodor and Offermaims 2008 Vosper 2009). 

Therefore, if we utilize the equation for the catabolism of carbohydrates, we can calculate a respiratory quotient ... 

Acetyl CoA is produced by the catabolism of carbohydrates, fats, and certain amino acids. The catabolism of fatty acids predominates over the catabolism of carbohydrates in certain illnesses, such as diabetes. When there is not enough oxaloacetate to react with the available CoA, a Claisen condensation of two acetyl CoA molecules produces acetoacetyl CoA. 

The Catabolism of Carbohydrates starts either with glycogen or with free glucose. In the first case, phosphorolysis yields the phosphate of glucose (glucose-l-P)... 

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