University:
Liberty University
Course:
BIOM 500 | Introduction to Biomedical Sciences
Academic year:

2024
Views:

412

Pages:

5
Author:

Sheha A.

Pentose Phosphate Pathway It is also called as Hexose Mono Phosphate Pathway and direct oxidative pathway of glucose. Definition: HMP shunt pathway is an alternative pathway of oxidation of glucose. It is a metabolic pathway that generates NADPH and pentose. It also plays an important role in glucose anabolism. Site: Tissue site: Liver, RBC, adrenal cortex, testis, mammary glands Intracellular site: cytosol Starting compounds: Glucose 6-phosphate Phases: Oxidative phase / Decarboxylative phase Non-oxidative phase / Regenerative phase Decarboxylative phase: Glucose 6-phosphate is oxidized to 6-phosphogluconolactone by NADP linked glucose 6-phosphate dehydrogenase. 1 molecule of NADPH is produced. Pentose Phosphate Pathway Regenerative phase: Ribulose 5-phosphate is converted to its isomer Ribose 5-phosphate by Ribulose 5-phosphate isomerase and some Ribulose 5-phosphate to Xylulose 5-phosphate by epimerase. Ribose 5-phosphate and Xylulose 5-phosphate are acted upon by Transketolase which transfers the keto group from Xylulose 5-phosphate to Ribose 5-phosphate. As a result, 7-carbon keto sugar Sedoheptulose 7-phosphate and 3-carbon aldehyde glyceraldehyde 3-phosphate are formed. Reaction requires the presence of TPP and Mg2+. Sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate are converted to fructose 6-phosphate and erythrose 4-phosphate by Transaldolase. Significance: NADPH produced during HMP pathway has following functions: NADPH is needed for the synthesis of fatty acid, cholesterol, and steroid hormones. In RBC's, NADPH is required to convert oxidized glutathione to reduced glutathione. Reduced glutathione removes the toxic H2O2. This is important because H2O2 otherwise could oxidize the RBC membrane and cause hemolysis. Thus, NADPH prevents RBC membrane against hemolysis and maintains integrity of RBC membrane. HMP shunt produces Ribose 5-phosphate which is required for nucleotide synthesis. Pentose Phosphate Pathway (a) Glucose 6-phosphate dehydrogenase Glucose 6-phosphate + NADP+ ↓ Mg2+ 6-phosphogluconolactone + NADPH + H+ (b) Gluconolactone hydrolase 6-phosphogluconolactone + H2O ↓ 6-phosphogluconic acid (c) 6-phosphogluconate dehydrogenase 6-phosphogluconic acid + NADP+ ↓ Mg2+ 3-keto-6-phosphogluconic acid + NADPH + H+ + CO2 (d) decarboxylase 3-keto-6-phosphogluconic acid ↓ Ribulose 5-phosphate (or pentose 5-phosphate) Ribulose 5-phosphate ↓ isomerase Xylulose 5-phosphate (e) Transketolase Xylulose 5-phosphate + Ribose 5-phosphate ↓ Glyceraldehyde 3-phosphate + Sedoheptulose 7-phosphate (f) Transaldolase Sedoheptulose 7-phosphate + Glyceraldehyde 3-phosphate ↓ Fructose 6-phosphate + Erythrose 4-phosphate Erythrose 4-phosphate + Fructose 6-phosphate ↓ Enter glycolytic pathway (Glyceraldehyde 3-phosphate)