Fatty Acid Oxidation and Ketone Bodies — Beta-Oxidation, Ketogenesis and MCAD Deficiency

Fatty acid oxidation breaks fatty acids down to acetyl-CoA to release energy. It occurs in three places: mitochondria (where only beta-oxidation occurs), peroxisomes (alpha- and beta-oxidation) and the endoplasmic reticulum (omega-oxidation). Beta-oxidation is a major source of energy between meals and during high-demand states such as exercise.

In these states epinephrine and glucagon raise the rate of lipolysis, releasing free fatty acids from adipose tissue. Fatty acid oxidation then supplies a large part of the energy needs of skeletal muscle, heart muscle and kidneys when glycogen and gluconeogenic precursors are scarce — and it spares muscle protein from breakdown.

The fate of the acetyl-CoA depends on the tissue. In skeletal and cardiac muscle it enters the TCA cycle and supplies ATP. In hepatocytes during prolonged fasting, when glycogen is depleted, it is used to synthesise ketone bodies.

A free fatty acid must first be activated by forming a thioester with CoA. This ATP-dependent step is carried out by acyl-CoA synthetases, which are site- and chain-length specific: long-chain acyl-CoA synthetase (for 12–20 carbon fatty acids) sits on the outer mitochondrial, peroxisomal and endoplasmic reticulum membranes; medium-chain synthetases are only in the mitochondrial matrix; the very long-chain synthetase is found only in peroxisomes.

Long-chain acyl-CoA cannot cross the inner mitochondrial membrane by itself, so it uses carnitine:

1. Carnitine palmitoyltransferase I (CPT I) on the outer mitochondrial membrane converts fatty acyl-CoA to fatty acylcarnitine.
2. Carnitine-acylcarnitine translocase moves acylcarnitine into the matrix in exchange for carnitine (an antiport).
3. CPT II converts acylcarnitine back to fatty acyl-CoA, trapping it in the matrix, and regenerates the carnitine pool.

All four steps occur in the mitochondrial matrix and repeat in a spiral. Each round removes two carbons as acetyl-CoA and yields one FAD(H2) and one NADH.

StatPearls states that each round of mitochondrial beta-oxidation yields 4 ATP equivalents from FAD(H2) and NADH, plus one acetyl-CoA. A 16-carbon fatty acid such as palmitate goes through 7 rounds and gives 8 acetyl-CoA.

• Unsaturated fatty acids (oleate 18:1, linoleate 18:2) have cis double bonds that must be isomerised to trans (enoyl-CoA isomerase) or reduced at the expense of NADPH (2,4-dienoyl-CoA reductase).
• Odd-chain fatty acids are oxidised like even-chain ones, but the last round gives acetyl-CoA plus propionyl-CoA (3 carbons), which can be converted to succinyl-CoA and enter the TCA cycle.
• Very long-chain fatty acids (24–26 carbons) are oxidised in peroxisomes. The first step uses an oxidase that passes electrons to oxygen to make hydrogen peroxide, rather than storing them in FAD(H2). Shortened products can be carried to mitochondria by carnitine for complete oxidation.
• Branched-chain fatty acids such as phytanic acid (a breakdown product of chlorophyll from plant food) undergo alpha-oxidation in peroxisomes: phytanoyl-CoA hydroxylase (gene PHYH) adds a hydroxyl group to the alpha carbon, forming pristanic acid, which then enters beta-oxidation.
• Omega-oxidation in the endoplasmic reticulum uses the cytochrome P450 system to convert fatty acids to dicarboxylic acids, which are more water soluble and excreted in urine. When beta-oxidation is blocked, omega-oxidation is up-regulated and dicarboxylic acids appear in blood and urine.

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common inherited defect of fatty acid oxidation (StatPearls). The block at the first beta-oxidation step means 6–8 carbon (medium-chain) acyl-CoAs accumulate. The liver cannot make enough acetyl-CoA, so the liver cannot generate enough acetyl-CoA from fat to make ketone bodies, and the fasting child runs short of glucose — hence hypoketotic hypoglycaemia.

Affected children are normal at birth. GeneReviews reports that symptoms appear with prolonged fasting (for example when night-time feeds are dropped) or during common infections that reduce appetite and raise energy needs. Untreated, severe hypoglycaemic episodes can cause vomiting, lethargy, seizures, coma and death. Metabolic decompensation can raise liver transaminases and cause hyperammonaemia and hepatomegaly.

During fasting, adipose tissue releases free fatty acids to the liver, where beta-oxidation generates large amounts of acetyl-CoA. The citric acid cycle cannot absorb it all because oxaloacetate is diverted to gluconeogenesis, so the excess is channelled into ketogenesis, in liver mitochondria.

1. Thiolase condenses two acetyl-CoA to acetoacetyl-CoA (the same enzyme that runs step 4 of beta-oxidation in reverse).
2. HMG-CoA synthase adds a third acetyl-CoA to form HMG-CoA; increased activity of this mitochondrial enzyme drives ketone synthesis when insulin is low.
3. HMG-CoA lyase cleaves HMG-CoA to acetoacetate and acetyl-CoA.
4. Acetoacetate is reduced to D-beta-hydroxybutyrate by D-beta-hydroxybutyrate dehydrogenase, or spontaneously decarboxylated to acetone and CO₂, which is exhaled.

The three ketone bodies are acetoacetate, beta-hydroxybutyrate and acetone; acetone is a minor, volatile by-product. Ketone bodies are exported to brain, heart, skeletal muscle and kidneys, where they are converted back to acetyl-CoA for ATP. Unlike long-chain fatty acids they cross the blood-brain barrier, making them the principal alternative cerebral fuel in prolonged fasting.

Regulation is largely hormonal. Insulin is the principal inhibitor; glucagon, cortisol, catecholamines and thyroid hormones promote ketogenesis by driving lipolysis through hormone-sensitive lipase. When insulin is low, hormone-sensitive lipase is released from inhibition, more fatty acids reach the liver, and acetyl-CoA carboxylase activity falls. Less malonyl-CoA means CPT I is disinhibited, fatty acid entry into mitochondria rises, and ketogenesis accelerates.

In diabetic ketoacidosis (DKA) absolute insulin deficiency causes unrestrained lipolysis and ketogenesis. Glucose typically rises well above 250 mg/dL, the diagnostic threshold quoted by StatPearls; acidic ketone bodies cause an anion gap metabolic acidosis, with dehydration from osmotic diuresis, nausea, vomiting, abdominal pain, Kussmaul respiration and a fruity breath. Revise related concepts in anion gap, Kussmaul breathing and diabetes mellitus complications.

1. Name the location. Beta-oxidation = mitochondrial matrix; alpha-oxidation and VLCFA oxidation = peroxisome; omega-oxidation = endoplasmic reticulum.
2. Name the control point. CPT I is rate-limiting and is inhibited by malonyl-CoA; HMG-CoA synthase is the mitochondrial enzyme whose activity drives ketone synthesis.
3. Match the disorder to the accumulated metabolite. MCAD = medium-chain acylcarnitine (C8) and dicarboxylic aciduria; Zellweger = VLCFA and phytanic acid; X-ALD = VLCFA; Refsum = phytanic acid.
4. Spot the clue 'hypoketotic'. Fasting hypoglycaemia with inappropriately low ketones points to a fatty acid oxidation defect, especially MCAD.
5. Use the thiophorase one-liner. The liver makes ketone bodies but cannot use them.