Glucose-l -phosphate

FIGURE 7.23 The starch phosphorylase reaction cleaves glucose residues from amy-lose, producing a-D-glucose-l-phosphate. 

Interestingly, in the absence of sucrose and fructose, sucrose phosphory-lase will catalyze the exchange of inorganic phosphate, P , into glucose-l-phos-phate. This reaction can be followed by using as a radioactive tracer and observing the appearance of P into glucose-l-phosphate ... 

Thus, in the presence of just Pi and glucose-l-phosphate, sucrose phosphorylase still catalyzes the second reaction and radioactive Pi is incorporated into glucose-l-phosphate over time. 

Maltose phosphorylase proceeds via a single-displacement reaction that necessarily requires the formation of a ternary maltose E Pi (or glucose E glucose-l-phosphate) complex for any reaction to occur. Exchange reactions are a characteristic of enzymes that obey double-displacement mechanisms at some point in their catalysis. 

Sugar nucleotides are formed from sugar-l-phosphates and nucleoside triphosphates by specific pyrophosphorylase enzymes (Figure 23.18). For example, UDP-glucose pyrophosphorylase catalyzes the formation of UDP-glucose from glucose-l-phosphate and uridine 5 -triphosphate ... 

The reaction proceeds via attack by a phosphate oxygen of glucose-l-phosphate on the a-phosphorus of UTP, with departure of the pyrophosphate anion. The reaction is a reversible one, but—as is the case for many biosynthetic reactions —it is driven forward by subsequent hydrolysis of pyrophosphate ... 

D-glucose-l-phosphate and D-fructose, Hassid, Doudoroff and Barker18 succeeded in crystallizing a non-reducing disaccharide which was indistinguishable from natural sucrose. It was thus that the first laboratory synthesis of sucrose was achieved. 

After removal of the toluene, the mixture was pasteurized at 80° for 5 minutes, cooled, adjusted to pH 7.8 and 2.5 volumes of 95% alcohol added. The mixture was allowed to remain at 4° for 3 hours and the precipitate, containing most of the inorganic and esterified phosphate, was removed by filtration and the alcohol was removed by distillation in vacuo at about 30°. The solution was then made up to 600 ml. and passed through columns of Amberlite IR-100 and Amberlite IR-4. This treatment removed all the electrolytes, including the remaining traces of n-glucose-l-phosphate. After washing the columns with water, the volume had increased to about three liters. The solution was concen-... 

The enzymatic synthesis of sucrose also throws light on the formation of the furanose form of fructose in the sucrose molecule. The fact that sucrose is directly formed from D-glucose-l-phosphate and D-fructose supports Isbell and Pigman s34 and Gottschalk s85 evidence that the latter monosaccharide occurs in solution in an equilibrium mixture of furanose and pyranose forms. This makes it unnecessary to postulate a special mechanism of stabilization of a five membered (furanose) ring before the formation of compound sugars containing the D-fructose molecule.86... 

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