S/A Lipid Synthesis

Question 1

Fatty acid synthesis and fatty acid breakdown occur by similar pathways. Describe, very briefly, four ways in which the synthetic and breakdown pathways differ.

Answer 1

Fatty acid synthesis (in any order)

(1) employs NADPH as reducing agent;
(2) involves an acyl group bound to a protein, ACP; (3) takes place in the cytosol of animals;
(4) involves the condensation of malonyl- and acetyl-groups;
(5) involves the formation of the D-β-hydroxyacyl derivative.

Fatty acid breakdown

(1) employs NAD+ as electron acceptor;
(2) involves acyl groups bound to coenzyme A;
(3) occurs in the mitochondrial matrix;
4) does not involve malonyl- derivatives;
(5) involves the L-stereoisomer of the β-hydroxyacyl derivative.

Question 2

Describe the mechanism for moving acetyl-CoA produced in the mitochondrial matrix into the cytosol for fatty acid synthesis.

Answer 2

cetyl-CoA in the mitochondrial matrix condenses with oxaloacetate to form citrate in a reaction catalyzed by citrate synthase. Citrate moves out of the matrix via the citrate transporter. Citrate in the cytosol is cleaved by citrate lyase, yielding acetyl-CoA and oxaloacetate. To complete the cycle, oxaloacetate in the cytosol is reduced to malate, which moves into the mitochondrial matrix on the malate-α-ketoglutarate transporter, and is converted to oxaloacetate in the matrix.

Question 3

why we require fats in our diets.

Answer 3

Dietary fats provide the linoleate and linolenate that we need (for eicosanoid synthesis) but cannot synthesize.

Question 4

Describe two basic strategies for activating precursors in the biosynthesis of phospholipids.

Answer 4

: (1) Activate the head group by attachment to CDP, as in CDP-choline, then displace CMP with the hydroxyl group of glycerol in diacylglycerol. (2) Activate diacylglycerol by the attachment of CDP, then displace CMP with the hydroxyl group of the head group.