larabinose

L-arabinose is an aldopentose sugar and the enantiomer of D-arabinose. It occurs in two cyclic forms, L-arabinofuranose and L-arabinopyranose, which interconvert in solution. In nature, L-arabinose is a component of plant polysaccharides such as arabinans, arabinogalactans, and arabinoxylans found in plant cell walls. Free L-arabinose is typically present only in small amounts in foods of plant origin.

Biochemistry and metabolism: In bacteria such as Escherichia coli, L-arabinose is metabolized via an inducible operon. L-arabinose isomerase (AraA) converts it to L-ribulose, which is phosphorylated by L-ribulokinase (AraB) to L-ribulose-5-phosphate. L-ribulose-5-phosphate 4-epimerase (AraD) then converts it to D-xylulose-5-phosphate, an intermediate of the pentose phosphate pathway. The araBAD operon coordinates regulation of these enzymes in response to arabinose availability and glucose repression. Humans lack a complete, efficient pathway to metabolize L-arabinose; it is poorly absorbed and largely excreted, with any minor metabolism largely attributable to gut microflora.

Applications: In molecular biology, L-arabinose is used to induce expression of genes under the control of the araBAD promoter in E. coli, enabling controlled production of recombinant proteins. It has also been studied as a potential competitive inhibitor of intestinal sucrase, which can reduce digestion of sucrose and attenuate postprandial glucose rises in some studies, though it is not a standard medical treatment.

Chemistry and safety: L-arabinose has the formula C5H10O5 and a molar mass of about 150.13 g/mol. It exists in equilibrium between pyranose and furanose forms in solution. It is generally regarded as safe at typical dietary levels, though high doses can cause gastrointestinal discomfort due to microbial fermentation.