Chapter 29.SE, Problem 22MP

Interpretation Introduction

Interpretation:

The pentose phosphate pathway for degrading sugars (see Problem 29-21) is the conversion of ribose 5-phosphate to glyceraldehyde 3-phosphate.

Concept introduction:

Reactions that make and break C-C-Bonds

Reactions that make and break carbon-carbon bonds form the basis of both degradative and biosynthetic metabolism.

The breakdown of glucose to CO₂ involves give such cleavages where as its synthesis involves the reverse process such reactions, considered from the synthetic direction involve addition of a nucleophilic carbanion to an electrophilic carbon atom. The most common electrophilic carbon atoms in such reactions are the sp² hybridized carbonyl atoms of aldehydes, ketones, etc. and CO₂

Stabilized carbanions must be generated to add to these electrophilic centres. Three examples are the aldol condensation (catalyzed eg by aldolase) claisen ester condensation (citrate synthase) and the decarboxylation of β-keto acids. (isocitrate dehydrogenase and fatly acid synthase).