Chapter 24, Problem 1P

Explaining the Stoichiometry of Fatty Acid Synthesis Carefully count and account for each of the atoms and charges in the equations for the synthesis of palmitoyl-CoA, the synthesis of malonyl-CoA, and the overall reaction for the synthesis of palmitoyl-CoA from acetyl-CoA.

Interpretation Introduction

Interpretation:

Each of the atoms and charges in the equations for the synthesis of palmitoyl CoA and the synthesis of malonyl CoA should be accounted and the overall reaction for the synthesis of palmitoyl CoA from Acetyl CoA should be discussed.

Concept Introduction:

The steps in the synthesis of palmitoyl CoA and the synthesis of malonyl CoA are as follows:

  $7{\text{ Acetyl-CoA + 7 HCO}}_3^{-}{\text{ + 7 ATP}}^{4-}\to {\text{ 7 malonyl-CoA}}^{-}{\text{ + 7 ADP}}^{3-}{\text{ + 7 P}}_i^{2-}{\text{ + 7 H}}^{+}$

  $\begin{array}{c}{\text{Acetyl-CoA + 7 malonyl-CoA}}^{-}{\text{ + 14 NADPH + 14 H}}^{+}\to {\text{ palmitoyl-CoA + 7 HCO}}_3^{-}\\ {\text{+ 14 NADP}}^{+}\\ \text{ + 7 CoASH}\end{array}$

The overall reaction for the synthesis of palmitoyl CoA from acetyl CoA is,

  $\begin{array}{l}{\text{8 Acetyl-CoA + 7 ATP}}^{4-}{\text{ + 14 NADPH + 7 H}}^{+}\to \\ {\text{ palmitoyl-CoA + 14 NADP}}^{+}{\text{ + 7 CoASH + 7 ADP}}^{3-}{\text{ + 7 P}}_i^2\end{array}$

Explanation of Solution

In the synthesis of malonyl CoA, the carbons in acetyl group of acetyl CoA is derived from glucose in glycolysis or side chains of amino acids. HCO₃- is produced by reacting CO₂ and H₂O which is catalyzed by carbonic anhydrase. ADP and P_i are the products of ATP hydrolysis. A carbon from bicarbonate ion is incorporated into malonyl CoA while removing a water molecule.

In the synthesis of palmitoyl CoA, acetyl CoA and malonyl CoA is reacted each other and as a result CO₂ is released. This CO₂ is hydrated and subsequently ionized forming HCO₃- ion. Therefore production of protons also occurs. This is why there is an amount protons with half of the amount of NADPH required present on left side of the equation. C-15 and C-16 of palmitoyl CoA are derived from acetyl CoA directly. Remaining carbons are derived indirectly from acetyl CoA via malonyl CoA.