Answer to Question #125286 in Biochemistry for Amuda Manikandan

Question #125286
In aqueous solution, almost all D-glucose molecules are in the pyranose form. Other aldoses have a greater proportion of molecules in the open-chain form. D-Glucose may have evolved to be the predominant hexose because it is less likely than its isomers to react with and damage cellular proteins. Explain why D-glucose reacts less than other aldoses with the amino groups of proteins.
Expert's answer

Glucose is reactive because of the presence of an aldehyde group in its open-chain form. The aldehyde group slowly condenses with amino groups to form Schiff-base adducts.

Glucose "(C_6H_{12}O_6)" is a hexose, a monosaccharide containing six carbon atoms. Glucose is an aldehyde (contains a -CHO group). Five of the carbons plus an oxygen atom form a loop called a "pyranose ring", the most stable form for six-carbon aldoses. In this ring, each carbon is linked to hydroxyl and hydrogen side groups with the exception of the fifth atom, which links to a "6^{th}" carbon atom outside the ring, forming a CH2OH group. This ring structure exists in equilibrium with a more reactive acyclic form, which makes up "0.0026" percent at "pH7"

Probably more important to advanced life is the low tendency of glucose, by comparison to other hexose sugars, to nonspecifically react with the amino groups of proteins. This reaction (glycosylation) reduces or destroys the function of many enzymes. The low rate of glycosylation is due to glucose's preference for the less reactive cyclic isomer. In respiration, through a series of enzyme-catalysed reactions, glucose is oxidized to eventually to form carbon dioxide and water, yielding energy, mostly in the form of ATP.

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