A-L-Arabinoside

Fig. 7.—Predicted (- -) and Fragment (—) Circular Dichroism Spectra /3-L-Arabinose Calculated from (a) a-D-Xylose, and (b) Methyl /3-L-Aiabinoside a-L-Arabinose Calculated from (c) j3-D-Xylose, and (d) Average Calculated for /3-L-Arabinose Methyl a-L-Arabinoside Calculated from (e) Methyl /3-D-Xyloside, and (f) Methyl /3-L-Arabinoside. (Redrawn from Ref. 6.)...

This enzyme [EC 3.2.1.55], also called arabinosidase, catalyzes the hydrolysis of terminal nonreducing a-L-arabinofuranoside residues in a-L-arabinosides. Actual substrates include a-L-arabinofuranosides, a-L-arabi-nans containing (1,3)- and/or (l,5)-linkages, arabinoxy-lans, and arabinogalactans. It should be noted that some /3-galactosidases and )3-D-fucosidases will also hydrolyze a-L-arabinosides. 

Hemicellulase Hydrolysis of P-l,4-glucans, a-L-arabinosides, P-D-mannosides, 1,3-P-D-xylans, and other polysaccharides, yielding polysaccharides of lower molecular weight. 

This enzyme catalyzes the hydrolysis of P D-galactosides and a-L-arabinosides. It is best known for its action in hydrolyzing lactose and is, therefore, also known as lactase. The enzyme is widely distributed and occurs in higher animals, bacteria, yeasts, and... 

Sodium ions are better activators for the hydrolysis of o-nitrophenyl /S-D-galactoside and o-nitrophenyl a-L-arabinoside, and K ions for that of lactose and p-nitrophenyl /3-o-galactoside (compare Ref. 20). More substrates and alkali ions will have to be studied before any correlation or generalization can be made. 

It has been reported that whereas in the presence of o-nitrophenyl /S-D-galactoside, it is possible to measure the enzyme activity even at pH 5, with o-nitrophenyl a-L-arabinoside, exact measurements were not possible below pH 6. This shows that the generally known stabilizing... 

A and B stand for the groups dissociating on the acidic and alkaline side of the pH optimum, respectively. Key C, o-nitrophenyl 0-o-galactoside D, lactose E, o-nitrophenyl a-L-arabinoside F, p-nitrophenyl p-n-galactoside. The values in brackets have been taken from the p -pH curve. 

Determination of the Rate-determining Step.—Hydrolysis of o- and p-nitrophenyl /3-D-galactoside and of o-nitrophenyl a-L-arabinoside with the /3-galactosidase of E. coli, ML 309, has been studied in a stopped-flow apparatus at high enzyme concentrations, in order to determine the rate-determining step in the following scheme of reaction (see also. Section II, 10). 

Problem 34.25 When either methyl a-L-arabinoside or methyl j3-D-xyloside is methylated, hydrolyzed, and then oxidized by nitric acid, there is obtained a trimethoxy-glutaric acid, (a) What ring size is indicated for these aldopentosides (b) Predict the products of HIO4 oxidation of each of these aldopentosides. 

Dimethyl 4,4 -dimethoxy-5,6 5, 6 -bis(methylenedioxy)biphenyl-2,2 -dicarboxylate, in H-10051 Luteolin 4 -0-a-L-Arabinoside, in T-10052 Luteohn 3 -0-jff-D-Xylopyranoside, in T-10052... 

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