Biochemistry: Transketolase and its Reactions

Transketolase and Transaldolase

• Transketolase is an enzyme requiring TPP (thiamine pyrophosphate) that transfers two-carbon units from ketose to aldose.
• It catalyzes a reaction where a two-carbon unit is transferred from xylulose-5-phosphate to ribose-5-phosphate.
• Transaldolase transfers three-carbon units from a ketose to an aldose.

Nonoxidative Phase of the Pathway

• The nonoxidative phase results in the synthesis of ribose-5-phosphate along with glycolytic intermediates glyceraldehyde-3-phosphate and fructose-6-phosphate.
• This phase is crucial for producing ribose-5-phosphate, a key building block for nucleotide synthesis.

Pentose Phosphate Pathway Overview

• Also known as the hexose monophosphate shunt, the pentose phosphate pathway is an alternative glucose oxidation pathway that does not generate ATP.
• Prominent products include NADPH and ribose-5-phosphate.
• Occurs in the cytoplasm and comprises two distinct phases: oxidative and nonoxidative.

Importance in Plants and Cells

• In plants, the pathway synthesizes glucose during the dark reactions of photosynthesis.
• The oxidative phase is highly active in red blood cells due to their high need for NADPH.
• G-6-PD regulates the pentose phosphate pathway, with activity inhibited by NADPH.
• Diets rich in carbohydrates enhance G-6-PD and phosphogluconate dehydrogenase synthesis.

Oxidative Phase Details

• The oxidative phase consists of three key reactions, converting glucose-6-phosphate to ribulose-5-phosphate and producing two molecules of NADPH.
• Glucose-6-phosphate is oxidized by glucose-6-phosphate dehydrogenase (G-6PD).
• First reaction products include 6-phosphogluconolactone and NADPH; 6-phosphogluconolactone is hydrolyzed to produce 6-phospho-D-gluconate.
• A second NADPH is produced during the oxidative decarboxylation of 6-phosphogluconate.
• This phase primarily supports lipid synthesis and helps protect against oxidative damage due to high levels of NADPH, which act as powerful antioxidants.

Non-Oxidative Phase Conversions

• Begins with the conversion of ribulose-5-phosphate to ribose-5-phosphate through isomerization catalyzed by ribulose-5-phosphate isomerase.
• Useful intermediates produced in this phase include ribose-5-phosphate, fructose-6-phosphate, and glyceraldehyde-3-phosphate.

This quiz focuses on the enzyme transketolase and its role in the transfer of carbon units between ketoses and aldoses. It covers key reactions including those catalyzed by transaldolase and the synthesis of important metabolites in the nonoxidative phase of the pentose phosphate pathway.