Pathways of Glucose Metabolism

The next reaction in the glycolysis pathway is catalyzed by phosphoglucoi-somerase and involves the isomerization of glucose-6-phosphate into fructose-6- 

COO c-o E) II CH, Phosphoe/w/- pyruvate 10 Oxidation in citric acid cycle  

PFK II and fructose-2,6-biphosphatase activities are carried by a single protein molecule, which is a homodimer with 470 amino acid residues in each subunit. The phosphatase activity resides in the C-terminal domain of each subunit, whereas the kinase activity is located in the N-terminal section. 

The enzyme that catalyzes the conversion of PEP to pyruvate is pyruvate kinase. Liver pyruvate kinase is stimulated allosterically by fructose-1,6-diphosphate, AMP, ADP, and glyceraldehyde-3-phosphate. It is inhibited by alanine, ATP, NADH, and, more importantly, by cAMP- and Ca2 calmodulin-controlled phosphorylation. High blood glucagon levels thus inhibit the activities of both PFK II and pyruvate kinase in the liver through phosphorylation. Transcription of pyruvate kinase is also decreased by glucagon and increased by insulin. Muscle pyruvate kinase is not subject to cAMP or Ca2+ regulation. The pyruvate kinase reaction is practically irreversible. 

LDH [molecular weight (MW) about 150,000] occurs in most tissues and consists of isoenzymes whose electrophoretic pattern is tissue specific. There are 

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JH Kinoshita, T Masurat, M Herfaut. (1955). Pathway of glucose metabolism in corneal epithelium. Science 122 72. 

Both G-6-PDH and 6-PGDH have been found in tissues of plants, microorganisms, and animals, suggesting the widespread importance of the oxidative pathway of glucose metabolism. 

A, The pentose phosphate pathway (PPP), also called the hexose monophosphate shunt, is an alternate pathway of glucose metabolism that supplies the NAD PH required by many biosynthetic pathways. 

The metabolism of free L-fucose (6-deoxy-L-galac-tose), which is present in the diet and is also generated by degradation of glycoproteins, resembles the Entner-Doudoroff pathway of glucose metabolism (Eq. 17-18). Similar degradative pathways act on D-arabinose and L-galactose.60... 

Hochachka, P.W. and Hayes, F.R. (1962). The effect of temperature acclimation on pathways of glucose metabolism in the trout. Canadian Journal of Zoology 20, 261-270. 

Figure 21.1. Pathway of glucose metabolism by homofermentative lactic acid bacteria.

Vitamin C is a vitamin for only a limited number of vertebrate species humans and the other primates, the guinea pig, bats, the passeriform birds, and most fishes. Most insects and invertehrates are also incapahle of ascorbate synthesis. Ascorbate is synthesized as an intermediate in the gulonolactone pathway of glucose metabolism in those vertebrate species for which ascorbate is a vitamin, one enzyme of the pathway, gulonolactone oxidase, is absent. 

Novello F, Me Lean P. The pentose phosphate pathway of glucose metabolism measurement of the non-oxidative reactions of the cycle. Biochem. J. 1968 107 775-791. 

Fig. 1. Pathways of glucose metabolism in eubacteria and eukaryotes. The three major catabolic pathways are the Embden-Meyerhof glycolytic sequence (solid lines), the Entner-Doudoroff pathway (heavy solid lines) and the pentose phosphate pathway (dashed lines). The sequence from glyceraldehyde 3-phosphate to pyruvate is common to all three pathways.

After fructose and galactose enter cells, they are phosphorylated on carbon 1 and converted to intermediates in pathways of glucose metabolism. 

Yl. Yardley, H. J., and Godfrey, G., Direct evidence for the hexose monophosphate pathway of glucose metabolism in skin. Biochem. J. 86, 101-103 (1963). 

SECTION 15.3 Alternative Pathways of Glucose Metabolism and Hexose Interconversions... 

Available from the Emden-Meyerhof pathway of glucose metabolism. 

Fig 9. Overview of the major pathways of glucose metabolism. Pathways for production of blood glucose are shown by dashed lines. FA = fatty acids TG = triacylglycerols OAA = oxaloacetate PEP = phosphoenolpyruvate UDP-G = UDP-glucose DHAP = dihydroxyacetone phosphate. 

Glucuronate, once formed, can reenter the pathways of glucose metabolism through reactions that eventually convert it to D-xylulose 5-phosphate, an intermediate of the pentose phosphate pathway. In most mammals other than humans, an intermediate of this pathway is the precursor of ascorbic acid (vitamin C). Humans, however, are deficient in this pathway and cannot synthesize vitamin C. 

I Figure 13.14 shows the relationship of gluconeogenesis to the other major pathways of glucose metabolism. 

Occurrence. Phosphorylated D-erythro-pentulose is an intermediate in the oxidative pathway of glucose metabolism by yeast or animal tissue and has been recognized as an early product of photosynthesis in plants (see Chapter XIII). 

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