HMP Shunt Pathway, Fructose, Galactose Metabolism MCQs

Glucose is aldose sugar that typically exists as a pyranose ring in a solution. Fructose is a ketose with a furanose ring in a solution. Glucose and Galactose differ in only configuration around carbon 4, they are C-4 epimers that are interconvertible by action of an epimerase. Alpha-amaylase does not produce monosaccharides. Carbohydrates exist in D-isomeric form that is found in biological system while amino acid found in L-Isomeric form. Reference: Harper's 32 nd edition, page no.154

D and L Isomerism [Enantiomers] Difference in the orientation of H and OH group around Penultimate Carbon atom results in two mirror images called D and L isomers (Enantiomers). The penultimate carbon atom is called Reference Carbon atom. The penultimate carbon atom in Glucose and Fructose is C-5. Most of the naturally occurring Monosaccharides are D isomers [Unlike Amino Acids, which are L isomers.] Examples of Enantiomers D Glucose and L Glucose D Fructose and L Fructose D Mannose and L Mannose D Glyceraldehyde and L Reference: Harper's 32 nd edition, Page no. 153

In Diabetes mellitus, in the lens by polyol pathway Glucose converted to Sorbitol by the enzyme Aldose reductase In galactosemia, Dulcitol or Galactitol is responsible for cataract Sorbitol, Dulcitol are sugar alcohols. Reference: Harper's 32 nd edition, Page no.205

Phosphofructokinases the pace-setting enzyme of glycolysis. PFK is inhibited by ATP and citrate, uses fructose 6-phosphate as substrate, and catalyzes a reaction that's far from equilibrium. The reactions activated by fructose 2,6-bisphosphate. Reference: Lippincoatt SAE/Page no.195 Reference: Harper's 32 nd edition

Ans. is > b' i.e., 2 [Ref : Harper 29"/e p. 129-130] In glycerophosphate shuttle, the mitochondrial enzyme is linked to respiratory chain (ETC) via a flavoprotein, So only 1.5 mol of ATP are produced (According to older calculations, 2 ATP mol of ATP are produced). In malate shuttle, the mitochondrial enzyme is linked to ETC via NAD, so 2.5 mol of ATP are produced (according to older calculations 3 mol of ATP are produced).

Steps releasing ATP at the level of substrate 1, 3 Bisphosphoglycerate to 3 Phosphoglycerate (1, 3 Bisphosphoglycerate Kinase) Phosphoenolpyruvate to Pyruvate. (Pyruvate Kinase) Succinyl CoA to Succinate (Succinate Thiokinase) Reference: Harper's 32 nd edition, Page no. 170,172

Carbohydrate absorption occurs at enterocytes of the upper region of small intestine villi. Fructose absoption is via the facilitated transporters GLUT 5 across the apical enterocytes membrane and GLUT 2 across the basolateral enterocytes. Glucose and galactose on the other hand are transported in to enterocytes on the carriers in combination with sodium ions. The energy requirement is provided by Na/K ATPase pump. Experimental conditions that collapse the sodium electrochemical gradient, hypoxia, or poisoning of the Na-K ATPase by quabain inhibit glucose but not fructose absorption. Reference: Harper's 32 nd edition, Page no. 538

Ans. is 'b' i.e., Fructose [Ref Harper's 28th /e p. 179)