Question

Lipid metabolism enzymes (Substrates and final products)

Question:

Digestion of macronutrients occurs throughout the length of the GI tract. List the enzymes involved in lipid digestion, the tissue source of these enzymes, where the enzymes function, and any regulatory hormones that play a role in modulating digestive enzyme secretion. For each of the enzymes listed, please indicate the primary substrate and final product of the reaction.

Answer 1

Lipid metabolic enzymes:

acetyl-CoA carboxylase (ACC), fatty-acid synthase (FAS) and carnitine palmitoyl transferase (CPT)

Stearoyl-CoA desaturase-1 (SCD1) regulates lipid oxidation by converting stearic acid (18:0) to oleic acid (18:1).

The saturated fatty acyl-CoAs are known to allosterically inhibit ACC1, whereas monounsaturated fatty acyl-CoAs are the preferred substrates for the lipid synthesis of triacylglycerol (TAG) in the endoplasmic reticulum (ER).

Malonyl-CoA and stearic acid reciprocally regulate the entry of acyl-CoA into mitochondria by modulating the activity of CPTA. Lysophosphatidic acid (LPA) and phosphatidic acid (PA) are synthesized in the ER; they are also produced in mitochondria and transported to the ER where the terminal enzymes for TAG synthesis are located.

Mitochondrial FAS (FAS II) and acyl-carrier protein (ACP) are involved in fatty-acid synthesis, but their role in lipid metabolism remains elusive. ACS, acyl-CoA synthase; AGPAT, acylglycerol-3-phosphate acyltransferase; CLS, cardiolipin (CL) synthase; DGAT, diacylglycerol (DAG) acyltransferase; GPAT, glycerol-3-phosphate (G-3-P) acyltransferase; MCD, malonyl-CoA decarboxylase; MGAT, monoacylglycerol (MAG) acyltransferase; PAP, phosphatidic-acid phosphohydrolase; PGPS, phosphatidylglycerophosphate (PGP) synthase; TAG, triacylglycerol; TCA, tricarboxylic-acid cycle.