Q29-5P - Organic Chemistry (Mcmurry)

Question

PROBLEM 29-5 Evidence for the role of acetate in fatty-acid biosynthesis comes from isotope labelling experiments. If acetate labelled within the methyl group (${}^{\mathbf{3}}\mathbf{CH}_{\mathbf{3}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}$) were incorporated into fatty acids, at what positions in the fatty-acid chain would you expect the ${}^{\mathbf{13}}\mathbf{C}$ label to appear?

Step-by-Step Solution

Answer

The ${}^{13}\mathrm{C}$ label is projected to emerge at places such as C-2, C-4, C-6, C-8, and so on.

Introduction to the Concept

Isotopic labelling (also known as isotopic labelling) is a technique for tracking the passage of an isotope (a neutron-counting atom) through a chemical, metabolic pathway, or cell.

Solution Explanation

The fact that many fatty acids have an even number of carbon atoms is a noteworthy trait. This happens because all fatty acids are made biosynthetically from acetyl COA by adding two carbon units to a developing chain in a sequential manner.

In the glycolysis process, acetyl COA is formed predominantly from the metabolic breakdown of carbohydrates.

In fact, isotone labelling tests have indicated that acetate plays an important role in fatty acid production.

Given that chain groups form when carbon atoms are added in even numbers, the overall effect of the eight steps in the fatty acid biosynthesis pathway is to unite two 2-carbon acetyl groups into a 4-carbon butyryl group. Condensation of the butyryl group with another malonyl ACP provides a 6-carbon unit, and shell repeats add two carbon atoms to each unit, resulting in a 16-carbon palmitoyl ACP. This addition mode; i.e. a two-unit increase in carbon chain elongation implies that if acetate labelled within the methyl group ${}^{13}\mathrm{CH}_3{\mathrm{CCO}}_2^{-}$ were incorporated into fatty acids, the ${}^{13}\mathrm{C}$ label would always appear in the even carbon atom number of the increasing chain, starting at pay C-2, then C-4, C-6, C-8, and so on.

Isotone labelling tests show that elongation of fatty acid chains occurs by two carbon units at a time; with an even carbon chain is forming most of the time.