# Free Fatty Acid Metabolism > Free fatty acid metabolism in MBBS Biochemistry: hormone-sensitive lipase, albumin transport, carnitine shuttle, beta-oxidation ATP yield. - Canonical URL: https://prepelephant.com/topics/mbbs/biochemistry/free-fatty-acid-metabolism - Exam / course: MBBS · Subject: Biochemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Free Fatty Acid Metabolism", PrepElephant, https://prepelephant.com/topics/mbbs/biochemistry/free-fatty-acid-metabolism ## Direct answer Between meals, adipose triglyceride is dismantled into non-esterified (free) fatty acids that enter plasma bound to albumin — up to roughly ten binding sites per molecule — and travel to heart, muscle and liver for beta-oxidation. Lipolysis is initiated by adipose triglyceride lipase and driven by hormone-sensitive lipase, which catecholamines activate through a cAMP-dependent phosphorylation cascade and insulin suppresses by rephosphorylation via phosphodiesterase 3B. Uptake needs CD36 and the fatty acid transport proteins; mitochondrial entry requires the carnitine shuttle, whose carnitine palmitoyltransferase-1 is inhibited by malonyl-CoA — the switch that diverts fatty acids to ketogenesis during fasting. Complete palmitate oxidation yields 8 acetyl-CoA and, by current estimates, about 106 ATP. ## What you must remember - **Control architecture:** catecholamines and glucagon activate hormone-sensitive lipase (cyclic AMP, protein kinase A, phosphorylation — the classic amplifying cascade); insulin shuts it off and simultaneously drives re-esterification through glycerol-3-phosphate from glucose uptake. - **Albumin carriage:** FFAs are the most abundant lipid fuel in plasma yet the least abundant by pool size because turnover is rapid (half-life of a few minutes); glycerol released with them cannot be re-esterified in adipose tissue, which lacks glycerol kinase. - **Carnitine shuttle numbers:** CPT-1 on the outer membrane is rate-limiting for mitochondrial import and is inhibited by malonyl-CoA (abundant in the fed state when acetyl-CoA carboxylase is active); CPT-2 reloads acyl-CoA inside. - **Yield arithmetic:** palmitate (C16) undergoes 7 spirals producing 7 FADH2, 7 NADH and 8 acetyl-CoA — approximately 106 ATP after costs, or 129 by older counts; quote "about 106, per current estimates" in vivas to be safe. - **Energy densities:** heart derives most resting ATP from fatty acid oxidation; brain cannot use FFAs (albumin-bound, no blood-brain transit) and switches to ketone bodies instead. - **Medium-chain acyl-CoA dehydrogenase deficiency** is the commonest beta-oxidation defect — hypoketotic hypoglycaemia with dicarboxylic aciduria, flagged by octanoylcarnitine (C8) on newborn screening. - **Clinical states of lipolysis:** uncontrolled diabetes and DKA (FFAs flood the liver, ketones pour out), stress and sepsis (myocardial FFA excess is arrhythmogenic), and rapid weight loss or glucocorticoid excess. ## Fasting physiology walked through, hour by hour After an overnight fast, insulin falls and glucagon and catecholamines rise; within minutes, hormone-sensitive lipase is phosphorylated and adipocyte triglyceride releases FFA and glycerol. The FFA-albumin complex reaches the liver, where two destinies compete: beta-oxidation to acetyl-CoA fuelling the Krebs cycle, or acetyl-CoA diverted — because oxaloacetate is being consumed for gluconeogenesis — to ketone bodies. By 3 days of fasting, hepatic ketogenesis supplies a growing share of the brain's fuel, sparing protein. Meanwhile glycerol from lipolysis enters gluconeogenesis as a genuine net glucose precursor (the fatty acid chain cannot yield glucose in humans, a two-mark certainty). Exercise rewires the same pathway: catecholamine-driven lipolysis, AMPK-mediated fatty acid entry and oxidation in muscle, with trained muscle oxidising fat at higher work rates. Now appreciate the diabetic failure mode: in DKA the identical machinery runs without brake — profound insulin deficiency means lipolysis is unrestrained, hepatic ketogenesis is massive, and the hypoketotic state of MCAD deficiency is its exact biochemical mirror image. ## Where students slip Two sentences fail vivas every year. "Fatty acids make glucose" — they do not in humans; even-chain acetyl-CoA has no net path to oxaloacetate, and only the odd-chain propionyl fragment is glucogenic. Second, "beta-oxidation needs carnitine" — only for long-chain fatty acids; medium and short chains and their synthetic inhibitors enter independently, which is exactly why MCAD defects present with hypoketotic rather than ketotic hypoglycaemia. A third nuance: brown adipose tissue in neonates oxidises fat uncoupled through UCP1 (thermogenesis), an Indian viva favourite because newborn hypothermia programmes quote it. ## Frequently asked questions ### Which enzyme is the principal regulated step of adipose lipolysis? Hormone-sensitive lipase, activated by catecholamines through cAMP and protein kinase A phosphorylation, suppressed by insulin. ### How do free fatty acids travel in plasma? Bound reversibly to albumin, which carries multiple fatty acid molecules per protein; only the tiny unbound fraction is metabolically active. ### Why does malonyl-CoA block fatty acid oxidation? It inhibits carnitine palmitoyltransferase-1, preventing long-chain acyl groups from entering mitochondria whenever synthesis is active in the fed state. ### Can fatty acid carbon become glucose? Not from even-chain fatty acids in humans, since acetyl-CoA cannot net-convert to oxaloacetate; glycerol backbones and odd-chain propionyl-CoA are glucogenic. ### What is the ATP yield of complete palmitate oxidation? Seven spirals yield 8 acetyl-CoA plus reduced cofactors, totalling about 106 ATP by current bioenergetic estimates.