Question

(i) Consider the mammalian biosynthesis of palmitoyl-ACP starting from the one acetylACP and seven malonyl-ACP. The “building blocks” (acetyl-ACP/malonyl-ACP) begin with more than twenty oxygens, yet the product (palmitoyl-ACP) includes only one. Where do the rest of the oxygens go?

(ii) The final product of the mammalian fatty acid synthase is palmitoyl-ACP, which contains one oxygen atom. Palmitic acid contains two oxygens. Where does this second oxygen come from?

Answer 1

(i) Consider the mammalian biosynthesis of palmitoyl-ACP starting from the one acetylACP and seven malonyl-ACP. The “building blocks” (acetyl-ACP/malonyl-ACP) begin with more than twenty oxygens, yet the product (palmitoyl-ACP) includes only one. Where do the rest of the oxygens go?

Addition of 2 carbon (acetyl-CoA) to the growing chain of fatty acid comes with loss of 3 oxygen (2 oxygen as CO2 in the β-ketoacyl synthase step and 1 oxygen as water in the β-hydroxylacyl-ACP dehydrase step).

For example, acetyl-CoA (1 oxygen) and malonyl-CoA (3 oxygen) forms via fatty acid synthase a butyryl-ACP which has only one oxygen while losing 3 oxygen in the form of CO2 and H2O.

In this way, all the oxygen from the building blocks (acetyl-CoA and malonyl-CoA) of fatty acids are released during fatty acid synthesis.

(ii) The final product of the mammalian fatty acid synthase is palmitoyl-ACP, which contains one oxygen atom. Palmitic acid contains two oxygens. Where does this second oxygen come from?

The second oxygen (the terminal oxygen) of palmitate comes from water during hydrolysis of plamitoyl-ACP into palmitate and ACP-SH.