Glucose transformations

Figure 16.12 Release of immobilized insulin in respond to addition of glucose. The disulfide bond is cleaved by electrons resulting from glucose transformation to gluconic acid by glucose oxidase. This approach requires a coenzyme flavin adenine dinucleotide (FAD)...

Figure 6.2 Time course of glucose transformation into fructose and 5-...

By adding glucosediphosphate, the coenzyme for the second reaction in excess, the over-all reaction depends on the galactose-glucose transformation and, therefore, the UDPGlucose concentration. The rate of the over-all reaction serves as a measure of coenzyme content. 

Since the first observation of the molybdic acid catalyzed interconversion of epi-meric aldoses it has been clear that the mechanism of the reaction is different to the enediol pathway of the Lobry de Bruyn-Alberda van Ekenstein (LdB-AvE) isomerization reaction of aldoses that provides 2-ketoses as primary major products [31]. Moreover, the interconverting aldoses do not incorporate deuterium if the molybdic acid catalyzed reaction is performed in deuterium oxide [32]. Furthermore, using specifically the tritium-labeled aldoses that were available in the 1970s,it has been proved that during the interconversion D-[l- H]glucose transforms to D-[2- H]mannose and vice versa. The results suggested that the process is intramolecular and involves a simultaneous transfer of the hydrogen atoms hnked to the carbon atoms C-1 and C-2. 

The cyclic AMP system is a nonspecific step in protein hormone action. This system is active in various target tissues (Table 41). The competence of the membrane receptor which transfers the signal from the hormone to cyclic AMP is, however, specific for every system. The biochemical basis for the action of this system can be illustrated by the example of the hormonal stimulation of the glycogen-glucose transformation. The reaction sequence in this case is as follows ... 

The biological transformations that involve ATP are both numerous and funda mental They include for example many phosphorylation reactions m which ATP trans fers one of its phosphate units to the —OH of another molecule These phosphoryla tions are catalyzed by enzymes called kinases An example is the first step m the metabolism of glucose... 

Xyhtol also is obtained by sodium borohydride reduction of D-xylonic acid y-lactone (32) and from glucose by a series of transformations through diacetone glucose (46). 

Increasingly, biochemical transformations are used to modify renewable resources into useful materials (see Microbial transformations). Fermentation (qv) to ethanol is the oldest of such conversions. Another example is the ceU-free enzyme catalyzed isomerization of glucose to fmctose for use as sweeteners (qv). The enzymatic hydrolysis of cellulose is a biochemical competitor for the acid catalyzed reaction. 

Enzymes are proteins of high molecular weight and possess exceptionally high catalytic properties. These are important to plant and animal life processes. An enzyme, E, is a protein or protein-like substance with catalytic properties. A substrate, S, is the substance that is chemically transformed at an accelerated rate because of the action of the enzyme on it. Most enzymes are normally named in terms of the reactions they catalyze. In practice, a suffice -ase is added to the substrate on which die enzyme acts. Eor example, die enzyme dial catalyzes die decomposition of urea is urease, the enzyme dial acts on uric acid is uricase, and die enzyme present in die micro-organism dial converts glucose to gluconolactone is glucose oxidase. The diree major types of enzyme reaction are ... 

Briefly summarized, S-1000 shows better resolution in the high dp range, whereas the CL 2B system resolves low dp components better. Due to these differences in separation performance, the degree of polymerization distribution and degree of polymerization average values for the same sample obtained from broad scb— nb/lcb-transformed dextran calibration yielded dp = 29,900 glucose units for the S-1000 system and dp = 21,100 glucose units for the CL 2B system (Fig. 16.18). 

T"he extraordinary ability of an enzyme to catalyze only one particular reaction is a quality known as specificity (Chapter 14). Specificity means an enzyme acts only on a specific substance, its substrate, invariably transforming it into a specific product. That is, an enzyme binds only certain compounds, and then, only a specific reaction ensues. Some enzymes show absolute specificity, catalyzing the transformation of only one specific substrate to yield a unique product. Other enzymes carry out a particular reaction but act on a class of compounds. For example, hexokinase (ATP hexose-6-phosphotransferase) will carry out the ATP-dependent phosphorylation of a number of hexoses at the 6-posi-tion, including glucose. 

Interestingly, phase-transfer catalysts including crown ethers have been used to promote enantioselective variations of Darzens condensation. Toke and coworkers showed that the novel 15-crown-5 catalyst derived from d-glucose 33 could promote the condensation between acetyl chloride 31 and benzaldehyde to give the epoxide in 49% yield and 71% A modified cinchoninium bromide was shown to act as an effective phase transfer catalyst for the transformation as well. ... 

Galactose, a constituent of the disaccharide lactose found in dairy products, is metabolized by a patiiwav that includes the isomerization of UDP-galactose to UDP-glucose. where UDP = uridylyl diphosphate. The enzyme responsible for the transformation uses NAD+ as cofactor. Propose a mechanism. 

Pyruvate, produced by catabolism of glucose (and by degradation of several amino acids), can undergo several further transformations depending on the conditions and on the organism. In the absence of oxygen, pyruvate can be either reduced by NADH to yield lactate [CHjCHfOHjCO - or, in yeast,... 

The process by which yeast breaks down glucose has been carefully studied by biochemists and the way in which this transformation occurs is now known in considerable detail. One of the reasons this process is so interesting is that a nearly identical process takes place in human muscle, in this case to furnish energy needed for muscular activity. 

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