Galactose enzymes

Figure 2B. Arrhenius plot for the reaction of fi-galactosidase with p-nitro-phenyl-fi-D-galactoside in 60% dimethyl sulfoxide, pH 7.0. Rate-determining step is formation of the galactose-enzyme intermediate. (18)...

Several epimeric pairs of monosaccharides are produced by epimerization of nucleotide sugars (Luckner, 1990). For example, UDP-D-glucose-4-epimerase converts UDP-D-glu-cose to UDP-D-galactose. Enzymes that produce the other epimers are also known. A C-2 epimerase may be involved in the interconversion of D-glucose and D-mannose (Karr, 1976). 

D-galactose. Enzymic hydrolysis of robinin yields a trisaccharide that must be L-rhamnosyl-D-galactosyl-L-rhamnose 130). 

TRISACCHARIDES. Trisaccharides, 3-sugar polymers, are not abundant in nature, but two trisaccharides—melezi-tose and raffinose—are found in limited amounts in certain plants. Melezitose, a component of sap in some coniferous plants, contains 2 molecules of glucose and 1 of fructose. Raffinose, which is found in sugar beets, molasses, beans, and cottonseed meal, consists of glucose, fructose, and galactose. Enzymes of the digestive tract are not capable of splitting melezitose and raffinose into monosaccharides. 

The reasons for the in vitro relief of polarity have recently been elucidated. A protein factor present in extracts from wildtype E. coli but not in extracts from E. coli suA restores polarity with both DNAs carrying insertions or amber codons (Wetekam and Ehring, 1973)- The polarity" factor is apparently lost in the preparation of the cell-free protein synthesis system. By means of RNA-directed in vitro synthesis of the galactose enzymes, the polarityrestoring activity has been shown to act also on preformed mRNA (Schumacher and Ehring, 1973)- These results clearly support the model of RNA degradation and are not compatible with a polarity model involving termination of transcription. 

Schumacher, G., Ehring, R. RNA-directed cell-free s3mthesis of the galactose enzymes of Escherichia coli. Molec. gen. Genet, in press (1973). 

Enzymes, measured in clinical laboratories, for which kits are available include y-glutamyl transferase (GGT), alanine transferase [9000-86-6] (ALT), aldolase, a-amylase [9000-90-2] aspartate aminotransferase [9000-97-9], creatine kinase and its isoenzymes, galactose-l-phosphate uridyl transferase, Hpase, malate dehydrogenase [9001 -64-3], 5 -nucleotidase, phosphohexose isomerase, and pymvate kinase [9001-59-6]. One example is the measurement of aspartate aminotransferase, where the reaction is followed by monitoring the loss of NADH ... 

FIGURE 6.20 A disaccharide of galactose and glucose is covalently linked to the 5-hydroxyl group of hydroxylysines in collagen by the combined action of the enzymes galactosyl transferase and glucosyl transferase. 

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. 

Galactose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized from UDP-glucose by galactose 4-epimerase, where UDP = uridylyl diphosphate (a ribonucleotide diphosphate Section 28.1). The enzyme requires NAD+ for activity (Section 17.7), but it is not a stoichiometric reactant. and NADH is not a final reaction product. Propose a mechanism. 

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