Carbohydrate metabolizing enzymes

Acid-soluble and acid-insoluble phosphates in ChloreUa vulgaris increased 2 to 4 fold, depending on the concentration of trichioroacetate, with the soluble phosphorus most affected. However, neither respiration nor the carbohydrate-metabolizing enzymes were particularly sensitive. Little change occurred in the hydrolyzable polysaccharides.168... 

Robinson, N and Preiss, J. 1987. Localization of carbohydrate metabolizing enzymes in guard cells of Commelina communis. Plant Physiol. 85, 360-364. 

Since the DNA concentration is about the same in psoriatic and normal epidermis (B14, M12), each cell must have a correspondingly decreased amount of structural or nonsoluble protein within it to counterbalance the increased amount of soluble protein. Within the psoriatic lesion there is more soluble protein per cell, with the ratio of carbohydrate metabolizing enzymes to the rest of these soluble proteins unaltered except for the increase in the pentose shunt activity which rises preferentially even in relation to the increased soluble protein. 

FIGURE 5.12 Enzymes are classified according to the specific biological reaction that they catalyze. Cells contain thousands of different enzymes. Two common examples drawn from carbohydrate metabolism are phos-phofrnctokinase (PFK), or, more precisely,... 

Aldol reactions occur in many biological pathways, but are particularly important in carbohydrate metabolism, where enzymes called aldolases catalyze the addition of a ketone enolate ion to an aldehvde. Aldolases occur in all organisms and are of two types. Type 1 aldolases occur primarily in animals and higher plants type II aldolases occur primarily in fungi and bacteria. Both types catalyze the same kind of reaction, but type 1 aldolases operate place through an enamine, while type II aldolases require a metal ion (usually 7n2+) as Lewis acid and operate through an enolate ion. 

Magnesium plays an important role in the transmission of nerve impulses. It is also important in the activity of many enzyme reactions, for example carbohydrate metabolism. Magnesium sulfate is used as replacement therapy in hypomagnesemia Magnesium sulfate (MgS04) is used in die prevention and control of seizures in obstetric patients with pregnancy-induced hypertension (PIH, also referred to as eclampsia and preeclampsia). It may also be added to TPN mixtures. 

Ghanges in the availability of substrates are responsible for most changes in metabolism either directly or indirectly acting via changes in hormone secretion. Three mechanisms are responsible for regulating the activity of enzymes in carbohydrate metabolism (1) changes in the rate of enzyme synthesis, (2) covalent modification by reversible phosphorylation, and (3) allosteric effects. 

The phosphorylation and dephosphorylation of seryl, threonyl, and tyrosyl residues regulate the activity of certain enzymes of lipid and carbohydrate metabolism and the properties of proteins that participate in signal transduction cascades. 

Thiamin has a central role in energy-yielding metabo-hsm, and especially the metabohsm of carbohydrate (Figure 45-9). Thiamin diphosphate is the coenzyme for three multi-enzyme complexes that catalyze oxidative decarboxylation reactions pymvate dehydrogenase in carbohydrate metabolism a-ketoglutarate dehydro-... 

Glucose phosphate isomerase (GPI) catalyzes the reversible interconversion of glucose-6-phosphate and fructose-6-phosphate. GPI plays an essential role in carbohydrate metabolism in all cells of the body. The substrates of this enzyme, ffuc-... 

Carbohydrate metabolism in the organism tissues encompasses enzymic processes leading either to the breakdown of carbohydrates (catabolic pathways), or to the synthesis thereof (anabolic pathways). Carbohydrate breakdown leads to energy release or intermediary products that are necessary for other biochemical processes. The carbohydrate synthesis serves for replenishment of polysaccharide reserve or for renewal of structural carbohydrates. The effectiveness of various routes of carbohydrate metabolism in tissues and organs is defined by the availability of appropriate enzymes in them. 

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

Enzyme Action. XLI. A Role of a-Ketoglutaric Acid in the Carbohydrate Metabolism of Wood-Destroying Molds. Arch. Biochem. 26, 375 (1950). 

Gil. Glock, G. E., and McLean, P., Levels of enzymes of the direct oxidative pathway of carbohydrate metabolism in the mammary gland of the rat. Biochim. et Biophys. Acta 12, 590 (1953). 

H2. Heller, P., Weinstein, H. G., West, M., and Zimmerman, H. J., Enzymes in anemia A study of abnormalities of several enzymes of carbohydrate metabolism in the plasma and erythrocytes in patients with anemia, with preliminary observations of bone marrow enzymes. Ann. Internal Med. 53, 898-913 (1900). 

Difficulties are encountered in the qualitative and quantitative analysis of carbohydrate mixtures because of the structural and chemical similarity of many of these compounds, particularly with respect to the stereoisomers of a particular carbohydrate. As a consequence, many chemical methods of analysis are unable to differentiate between different carbohydrates. Analytical specificity may be improved by the preliminary separation of the components of the mixture using a chromatographic technique prior to quantitation and techniques such as gas-liquid and liquid chromatography are particularly useful. However, the availability of purified preparations of many enzymes primarily involved in carbohydrate metabolism has resulted in the development of many relatively simple methods of analysis which have the required specificity and high sensitivity and use less toxic reagents. 

The availability of isotopes has made it possible to complete the descriptions of the steric course of most of the individual reactions of carbohydrate metabolism and steroid metabolism and many of the reactions of fat and amino acid metabolism. The subject has been covered from various angles in several chapters of the third edition of the Enzymes, particularly in the one by Popjack b, in a comprehensive treatise 2>3>, and in numerous recent reviews 4 12>. The wealth of available detail defies any attempt to be complete. I will try, rather, to describe trends in current experimentation, and to fit these trends into historical perspective. In so doing, I will select examples rather arbitrarily, entirely out of my own interests, and I beg the reader s indulgence for this bias. 

The calorific capacity of amino acids is comparable to that of carbohydrates so despite their prime importance in maintaining structural integrity of cells as proteins, amino acids may be used as fuels especially during times when carbohydrate metabolism is compromised, for example, starvation or prolonged vigorous exercise. Muscle and liver are particularly important in the metabolism of amino acids as both have transaminase enzymes (see Figures 6.2 and 6.3 and Section 6.4.2) which convert the carbon skeletons of several different amino acids into intermediates of glycolysis (e.g. pyruvate) or the TCA cycle (e.g. oxaloacetate). Not all amino acids are catabolized to the same extent... 

There are data from animal studies in mice, rats, and pigs that indicate that both carbohydrate metabolism and lipid metabolism may be affected by exposure to heptachlor or heptachlor epoxide (Enan et al. 1982 Halacka et al. 1974 Kacew and Singhal 1973 Pelikan 1971). Alterations in gluconeogenic enzymes and an increase in cellular steatosis in the liver have been reported. Granulomas and fibrotic liver have also been observed. In addition, hepatocellular carcinoma was identified as causally related to heptachlor in the diet in a mouse study conducted by the National Cancer Institute (NCI 1977). The existing evidence suggests that heptachlor and heptachlor epoxide are hepatic toxicants. 

Thiamine diphosphate (TDP) is an essential coenzyme in carbohydrate metabolism. TDP-dependent enzymes catalyze carbon-carbon bond-breaking and -forming reactions such as a-keto acid decarboxylations (oxidative and non-oxidative) and condensations, as well as ketol transfers (trans- and phospho-ketolation). Some of these processes are illustrated in Fig. 12. 

Thiamine pyrophosphate is a coenzyme for several enzymes involved in carbohydrate metabolism. These enzymes either catalyze the decarboxylation of oi-keto acids or the rearrangement of the carbon skeletons of certain sugars. A particularly important example is provided by the conversion of pyruvic acid, an oi-keto acid, to acetic acid. The pyruvate dehydrogenase complex catalyzes this reaction. This is the key reaction that links the degradation of sugars to the citric acid cycle and fatty acid synthesis (chapters 16 and 18) ... 

Palsamy P, Subramanian S. (2009) Modulatory effects of resveratrol on attenuating the key enzymes activities of carbohydrate metabolism in strep-tozotocin-nicotinamide-induced diabetic rats. Chem Biol Interact 179 356-362. 

Both the aldol and reverse aldol reactions are encountered in carbohydrate metabolic pathways in biochemistry (see Chapter 15). In fact, one reversible transformation can be utilized in either carbohydrate biosynthesis or carbohydrate degradation, according to a cell s particular requirement. o-Fructose 1,6-diphosphate is produced during carbohydrate biosynthesis by an aldol reaction between dihydroxyacetone phosphate, which acts as the enolate anion nucleophile, and o-glyceraldehyde 3-phosphate, which acts as the carbonyl electrophile these two starting materials are also interconvertible through keto-enol tautomerism, as seen earlier (see Section 10.1). The biosynthetic reaction may be simplihed mechanistically as a standard mixed aldol reaction, where the nature of the substrates and their mode of coupling are dictated by the enzyme. The enzyme is actually called aldolase. 

In Box 10.4 we saw that an aldol-like reaction could be used to rationalize the biochemical conversion of dihydroxyacetone phosphate (nucleophile) and glyceraldehyde 3-phosphate (electrophile) into fructose 1,6-diphosphate by the enzyme aldolase during carbohydrate biosynthesis. The reverse reaction, used in the glycolytic pathway for carbohydrate metabolism, was formulated as a reverse aldol reaction. 

---