Content

Location: cytosol. Two phases. Preparatory phase (spends 2 ATP) 1. Hexokinase (glucokinase in liver) phosphorylates glucose to glucose 6-phosphate. 2. Isomerase gives fructose 6-phosphate. 3. Phosphofructokinase-1 (PFK-1) gives fructose 1,6-bisphosphate. This is the main control point. 4. Aldolase splits it into DHAP and glyceraldehyde 3-phosphate, which are interconverted by triose phosphate isomerase. Payoff phase (per triose) 5. Glyceraldehyde 3-phosphate dehydrogenase makes 1,3-bisphosphoglycerate and NADH. 6. Phosphoglycerate kinase makes 3-phosphoglycerate and ATP (substrate-level phosphorylation). 7. Mutase and enolase give phosphoenolpyruvate. 8. Pyruvate kinase gives pyruvate and ATP. Balance: glucose + 2 NAD+ + 2 ADP + 2 Pi -> 2 pyruvate + 2 NADH + 2 ATP + 2 H2O + 2 H+. Exam trap: 4 ATP are made and 2 are spent, so the net gain is 2 ATP. Clinical tip: pyruvate kinase deficiency causes haemolytic anaemia, because red cells depend entirely on glycolysis.

Galactose: galactokinase makes galactose 1-phosphate, which is converted to UDP-galactose and then to UDP-glucose and glucose 1-phosphate. Galactosaemia results from a defect in galactose 1-phosphate uridyltransferase and causes cataract, liver damage and intellectual disability in infants fed milk. Fructose: in liver, fructokinase makes fructose 1-phosphate, which aldolase B splits into DHAP and glyceraldehyde. This bypasses the PFK-1 control step, so fructose is metabolised rapidly. Aldolase B deficiency causes hereditary fructose intolerance.

Site: mainly liver, also kidney cortex. Precursors: lactate, glycerol, glucogenic amino acids (alanine especially) and propionate. Fatty acids cannot make net glucose in humans. The bypass reactions 1. Pyruvate to phosphoenolpyruvate: pyruvate carboxylase (mitochondria, needs biotin) makes oxaloacetate, then PEP carboxykinase. 2. Fructose 1,6-bisphosphate to fructose 6-phosphate: fructose 1,6-bisphosphatase. 3. Glucose 6-phosphate to glucose: glucose 6-phosphatase (liver, in the endoplasmic reticulum). Cost: 4 ATP and 2 GTP plus 2 NADH per glucose. It is expensive but essential during fasting to feed the brain and red cells. Clinical tip: glucose 6-phosphatase deficiency (von Gierke disease) causes severe fasting hypoglycaemia and hepatomegaly.

Glycolysis - Hexokinase is inhibited by glucose 6-phosphate. Liver glucokinase has a high Km and is not inhibited, so the liver acts as a glucose sensor. - PFK-1 is inhibited by ATP and citrate and activated by AMP and fructose 2,6-bisphosphate. - Pyruvate kinase is activated by fructose 1,6-bisphosphate and, in liver, inhibited by phosphorylation (glucagon) and by alanine. Gluconeogenesis - Fructose 2,6-bisphosphate inhibits fructose 1,6-bisphosphatase and activates PFK-1. Insulin raises it and glucagon lowers it. - Acetyl-CoA activates pyruvate carboxylase. Hormones: insulin favours glycolysis, glucagon favours gluconeogenesis in liver. Glucose transporters: GLUT1 (most cells), GLUT2 (liver, pancreatic beta cell, high Km), GLUT3 (neurons), GLUT4 (muscle and fat, insulin dependent), GLUT5 (fructose).

Location: cytosol; active in liver, adipose tissue, adrenal cortex, red cells and dividing cells. Oxidative phase: glucose 6-phosphate dehydrogenase (the regulated, rate-limiting step) converts glucose 6-phosphate to 6-phosphogluconolactone and generates NADPH. Further steps give ribulose 5-phosphate and a second NADPH plus CO2. Non-oxidative phase: reversible transketolase and transaldolase reactions interconvert pentoses and glycolytic intermediates, so the cell can make ribose when needed or recycle it into glycolysis. Clinical tip: G6PD deficiency leaves red cells unable to make enough NADPH to keep glutathione reduced. Oxidant drugs, fava beans and infections cause haemolysis.

1. How many ATP are spent and made in glycolysis? What is the net yield? 2. Which glycolytic step is the main control point and what regulates it? 3. Name the three irreversible steps of glycolysis and the enzymes that bypass them in gluconeogenesis. 4. Why is glucokinase useful in the liver? 5. Which products does the pentose phosphate pathway supply? 6. Which precursors cannot give net glucose?

1. 2 ATP are spent and 4 made, so the net is 2 ATP (plus 2 NADH). 2. PFK-1. ATP and citrate inhibit; AMP and fructose 2,6-bisphosphate activate. 3. Hexokinase (glucose 6-phosphatase), PFK-1 (fructose 1,6-bisphosphatase), pyruvate kinase (pyruvate carboxylase plus PEP carboxykinase). 4. Its high Km lets the liver take up glucose only when blood glucose is high, and it is not inhibited by glucose 6-phosphate. 5. NADPH and ribose 5-phosphate. 6. Fatty acids (even-chain), because acetyl-CoA cannot be converted to pyruvate.

Breakdown (glycogenolysis) 1. Glycogen phosphorylase removes glucose from non-reducing ends as glucose 1-phosphate (uses phosphate, not water). It needs pyridoxal phosphate. 2. The debranching enzyme moves three residues from a branch and cuts the alpha(1-6) bond releasing free glucose. 3. Phosphoglucomutase converts glucose 1-phosphate to glucose 6-phosphate. 4. Liver has glucose 6-phosphatase and releases free glucose. Muscle lacks it and uses the glucose 6-phosphate itself. Synthesis (glycogenesis) 1. UDP-glucose pyrophosphorylase makes UDP-glucose. 2. Glycogenin primes the chain. 3. Glycogen synthase extends alpha(1-4) chains. 4. The branching enzyme makes alpha(1-6) branches. Clinical tip: glycogen storage diseases include von Gierke (glucose 6-phosphatase), Pompe (lysosomal alpha-glucosidase), Cori (debranching enzyme) and McArdle (muscle phosphorylase, exercise cramps).

Breakdown is activated by glucagon (liver) and adrenaline (liver and muscle) through cyclic AMP and protein kinase A, which activates phosphorylase kinase, which converts phosphorylase b to the active a form. In muscle, calcium and AMP also activate phosphorylase. Synthesis is favoured by insulin, which activates protein phosphatase 1. Phosphorylation inactivates glycogen synthase, and dephosphorylation activates it. So the same kinase cascade turns degradation on and synthesis off. Glucose itself, in liver, binds phosphorylase a and promotes its dephosphorylation. Exam trap: liver glycogen serves the whole body, muscle glycogen serves only the muscle.

1. Which enzyme removes glucose from glycogen and what is the product? 2. What does the debranching enzyme do? 3. Why can muscle not release glucose into the blood? 4. Which hormones activate phosphorylase? 5. Which enzyme deficiency causes McArdle disease?

1. Glycogen phosphorylase, giving glucose 1-phosphate. 2. It transfers three residues of a branch to the main chain and hydrolyses the alpha(1-6) bond. 3. Muscle lacks glucose 6-phosphatase. 4. Glucagon (liver) and adrenaline. 5. Muscle glycogen phosphorylase.

Anaerobic: lactate dehydrogenase converts pyruvate to lactate and regenerates NAD+, so glycolysis continues. Red cells and exercising muscle rely on this. Yeast makes ethanol and CO2. Cori cycle: muscle lactate goes to the liver, which converts it to glucose and returns it to muscle. Warburg effect: many tumour cells ferment glucose to lactate even with oxygen available. Aerobic: the pyruvate dehydrogenase complex (in the mitochondrial matrix) converts pyruvate to acetyl-CoA, CO2 and NADH. It needs five cofactors: thiamine pyrophosphate, lipoate, CoA, FAD and NAD+. It is inhibited by acetyl-CoA, NADH and ATP, and by phosphorylation. Clinical tip: thiamine deficiency impairs pyruvate dehydrogenase and causes lactic acidosis.

Location: mitochondrial matrix. 1. Acetyl-CoA + oxaloacetate -> citrate (citrate synthase). 2. Citrate -> isocitrate (aconitase). 3. Isocitrate -> alpha-ketoglutarate + CO2 + NADH (isocitrate dehydrogenase, main regulated step). 4. Alpha-ketoglutarate -> succinyl-CoA + CO2 + NADH (alpha-ketoglutarate dehydrogenase). 5. Succinyl-CoA -> succinate + GTP (succinyl-CoA synthetase). 6. Succinate -> fumarate + FADH2 (succinate dehydrogenase, also complex II). 7. Fumarate -> malate -> oxaloacetate + NADH. Per turn: 3 NADH, 1 FADH2, 1 GTP and 2 CO2. The cycle is amphibolic: it supplies intermediates for biosynthesis (citrate for fatty acids, oxaloacetate for glucose, alpha-ketoglutarate for amino acids). Regulation: ATP and NADH inhibit, ADP and calcium activate, at citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase.

Complexes in the inner mitochondrial membrane - Complex I (NADH dehydrogenase): pumps 4 H+. - Complex II (succinate dehydrogenase): feeds FADH2 electrons, no proton pumping. - Coenzyme Q (ubiquinone) carries electrons to complex III (pumps 4 H+ ), cytochrome c carries them to complex IV (cytochrome oxidase, pumps 2 H+), which reduces O2 to water. Chemiosmotic model (Mitchell): the proton gradient (proton-motive force) is the energy store. ATP synthase (F0 channel and F1 catalytic head) makes ATP as protons flow back. Yield: about 2.5 ATP per NADH and 1.5 per FADH2. Complete oxidation of one glucose gives about 30-32 ATP. Regulation: respiratory control, meaning that ADP availability limits the rate. Uncouplers (2,4-dinitrophenol, thermogenin in brown fat) dissipate the gradient and produce heat. Inhibitors: rotenone (I), antimycin A (III), cyanide and carbon monoxide (IV), oligomycin (ATP synthase). Clinical tip: cyanide poisoning blocks cytochrome oxidase, and cells die of energy failure despite adequate oxygen.

1. Why does lactate form in an exercising muscle? 2. List the cofactors of pyruvate dehydrogenase. 3. Give the products of one turn of the Krebs cycle. 4. What is the proton-motive force? 5. Which complex does not pump protons? 6. How does an uncoupler affect ATP synthesis and heat?

1. To regenerate NAD+ so glycolysis can continue when oxygen supply is limited. 2. Thiamine pyrophosphate, lipoate, CoA, FAD and NAD+. 3. 3 NADH, 1 FADH2, 1 GTP, 2 CO2. 4. The electrochemical proton gradient across the inner mitochondrial membrane that drives ATP synthase. 5. Complex II. 6. It dissipates the gradient, so less ATP is made and more energy is released as heat.

PRPP (phosphoribosyl pyrophosphate) is made from ribose 5-phosphate. Purines: built atom by atom on PRPP, giving inosine monophosphate (IMP), which branches to AMP and GMP. Precursors: glycine, glutamine, aspartate, formate (from tetrahydrofolate) and CO2. The committed step is PRPP amidotransferase, inhibited by AMP, GMP and IMP. Pyrimidines: carbamoyl phosphate synthetase II (cytosol) and aspartate give orotate, which is attached to PRPP to make UMP. UMP gives UTP and CTP. Regulation is at carbamoyl phosphate synthetase II, inhibited by UTP. Deoxyribonucleotides: ribonucleotide reductase converts ribonucleoside diphosphates to deoxy forms. Thymidylate synthase (using methylenetetrahydrofolate) converts dUMP to dTMP. Clinical tip: hereditary orotic aciduria is a defect of UMP synthesis.

- Methotrexate inhibits dihydrofolate reductase and so depletes the folate needed for thymidylate and purines. - 5-Fluorouracil inhibits thymidylate synthase. - Hydroxyurea inhibits ribonucleotide reductase. - Mycophenolate inhibits IMP dehydrogenase, limiting guanine nucleotides in lymphocytes. - Azathioprine and 6-mercaptopurine are purine analogues.

Salvage: HGPRT converts hypoxanthine and guanine back to IMP and GMP using PRPP. APRT does the same for adenine. Complete HGPRT deficiency (Lesch-Nyhan syndrome) causes hyperuricaemia, self-mutilation and intellectual disability. Degradation - Purines are broken down to hypoxanthine, xanthine and finally uric acid by xanthine oxidase. Excess uric acid crystals cause gout. Allopurinol inhibits xanthine oxidase. - Pyrimidines are broken down to soluble products (beta-alanine, beta-aminoisobutyrate, CO2, ammonia). Clinical tip: adenosine deaminase deficiency causes severe combined immunodeficiency.

1. What is PRPP and which pathways use it? 2. Which enzyme catalyses the committed step of purine synthesis and what inhibits it? 3. How does methotrexate stop cell division? 4. What is the enzyme defect in Lesch-Nyhan syndrome? 5. What is the final product of human purine degradation? How does allopurinol act?

1. Phosphoribosyl pyrophosphate, made from ribose 5-phosphate. It is used in purine and pyrimidine de novo synthesis and in salvage. 2. PRPP amidotransferase, inhibited by AMP, GMP and IMP. 3. It inhibits dihydrofolate reductase, so the cell lacks tetrahydrofolate for thymidylate and purine synthesis. 4. HGPRT deficiency. 5. Uric acid. Allopurinol inhibits xanthine oxidase.

Sources of the pool: dietary protein, breakdown of body proteins, and synthesis of non-essential amino acids. Fates: (1) synthesis of body proteins, (2) synthesis of nitrogen compounds (haem, creatine, neurotransmitters, nucleotides, glutathione, nitric oxide), (3) conversion of the carbon skeleton to glucose or fat, (4) oxidation for energy, mainly in fasting. Essential amino acids (must come from the diet): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine. Arginine is conditionally essential. Non-essential amino acids can be made from intermediates of glycolysis and the Krebs cycle. Nitrogen balance: intake minus excretion. Positive in growth and pregnancy, negative in starvation, illness and trauma.

Transamination: aminotransferases (ALT and AST) move the amino group from an amino acid to alpha-ketoglutarate, giving glutamate. They need pyridoxal phosphate (vitamin B6). Serum ALT and AST rise in liver damage. Oxidative deamination: glutamate dehydrogenase releases ammonia from glutamate in the liver mitochondria. Transport of ammonia in blood: as glutamine (from glutamine synthetase) and as alanine (glucose-alanine cycle from muscle). Urea cycle (liver; partly mitochondrial, partly cytosolic) 1. Carbamoyl phosphate synthetase I makes carbamoyl phosphate from ammonia and CO2 (needs N-acetylglutamate as activator). 2. Ornithine transcarbamylase makes citrulline. 3. Argininosuccinate synthetase adds aspartate. 4. Argininosuccinate lyase releases fumarate and arginine. 5. Arginase cleaves arginine to urea and ornithine. Cost: 4 high-energy phosphate bonds per urea. Clinical tip: urea cycle defects and liver failure cause hyperammonaemia with confusion, tremor and coma. Treatment: low protein diet, lactulose, sodium benzoate.

- Glucogenic amino acids give pyruvate or Krebs cycle intermediates and can form glucose: alanine, serine, glycine, cysteine (pyruvate); aspartate, asparagine (oxaloacetate); glutamate, glutamine, proline, arginine, histidine (alpha-ketoglutarate); valine, methionine, isoleucine (succinyl-CoA); phenylalanine and tyrosine (fumarate). - Ketogenic only: leucine and lysine (acetyl-CoA or acetoacetate). - Both: isoleucine, phenylalanine, tyrosine, tryptophan, threonine. Inherited defects: phenylketonuria (phenylalanine hydroxylase), maple syrup urine disease (branched-chain alpha-ketoacid dehydrogenase; leucine, isoleucine, valine accumulate), alkaptonuria (homogentisate oxidase), homocystinuria (cystathionine beta-synthase).

1. List the essential amino acids. 2. Which coenzyme do aminotransferases need? 3. Why is alanine important in transport of nitrogen from muscle to liver? 4. Name the first and the last reaction of the urea cycle. 5. Which amino acids are purely ketogenic? 6. Which enzyme is missing in phenylketonuria?

1. Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine. 2. Pyridoxal phosphate (vitamin B6). 3. Muscle transfers amino groups to pyruvate forming alanine, which the liver converts back to pyruvate for glucose and to urea for excretion. 4. First: carbamoyl phosphate synthetase I. Last: arginase (arginine to urea and ornithine). 5. Leucine and lysine. 6. Phenylalanine hydroxylase.

Mobilisation: hormone-sensitive lipase in adipocytes (activated by glucagon and adrenaline, inhibited by insulin) releases fatty acids that travel bound to albumin. Activation: acyl-CoA synthetase in the cytosol makes fatty acyl-CoA (uses ATP). Transport: carnitine acyltransferase I (CPT-1, outer membrane; inhibited by malonyl-CoA) exchanges CoA for carnitine; a translocase carries acylcarnitine in, and CPT-2 restores acyl-CoA in the matrix. This is the rate-limiting step. Beta-oxidation in the matrix repeats four steps: dehydrogenation (FAD), hydration, dehydrogenation (NAD+), thiolysis giving acetyl-CoA and an acyl-CoA two carbons shorter. Yield example: palmitate (16 C) needs 7 cycles, giving 8 acetyl-CoA, 7 FADH2 and 7 NADH: about 106 ATP net (activation costs 2 ATP equivalents). Odd-chain fatty acids give propionyl-CoA, converted to succinyl-CoA (biotin and vitamin B12 needed). Clinical tip: carnitine deficiency and MCAD deficiency cause hypoketotic hypoglycaemia in fasting.

Ketone bodies: acetoacetate, D-beta-hydroxybutyrate and acetone (from spontaneous decarboxylation). Synthesis (liver mitochondria): two acetyl-CoA -> acetoacetyl-CoA (thiolase) -> HMG-CoA (HMG-CoA synthase, the regulated step) -> acetoacetate (HMG-CoA lyase). Beta-hydroxybutyrate is made by reduction. Why they form: in fasting or untreated diabetes, oxaloacetate is used for gluconeogenesis, so acetyl-CoA cannot enter the Krebs cycle and is diverted. Use: extrahepatic tissues convert acetoacetate to acetoacetyl-CoA with the enzyme thiophorase (succinyl-CoA transferase), which the liver lacks, so the liver cannot use ketone bodies. The brain gets up to about two thirds of its energy from ketones in prolonged starvation. Clinical tip: diabetic ketoacidosis has high ketones, low pH and a fruity breath from acetone.

Where and when: cytosol of liver and adipose tissue (and mammary gland), stimulated by insulin, when carbohydrate is abundant. Acetyl-CoA is made in mitochondria and cannot cross the membrane, so it leaves as citrate and is regenerated in the cytosol by ATP-citrate lyase. Acetyl-CoA carboxylase (biotin) makes malonyl-CoA: this is the committed and regulated step (activated by citrate and insulin, inhibited by palmitoyl-CoA and by phosphorylation from glucagon and AMP kinase). Fatty acid synthase (a multi-enzyme complex) adds two-carbon units from malonyl-CoA in cycles of condensation, reduction, dehydration and reduction, using NADPH, until palmitate (16:0) is formed. NADPH comes from the pentose phosphate pathway and malic enzyme. Elongation and desaturation happen in the endoplasmic reticulum. Triacylglycerols are assembled and exported as VLDL. Exam trap: malonyl-CoA inhibits CPT-1, so synthesis and oxidation are not active together.

1. Which enzyme controls fatty acid entry into mitochondria and what inhibits it? 2. How many acetyl-CoA and NADH are made from palmitate? 3. Why can the liver not use ketone bodies? 4. What is the committed step of fatty acid synthesis? 5. Give two sources of NADPH.

1. CPT-1, inhibited by malonyl-CoA. 2. 8 acetyl-CoA, 7 NADH (and 7 FADH2). 3. It lacks thiophorase (succinyl-CoA:acetoacetate CoA transferase). 4. Acetyl-CoA carboxylase producing malonyl-CoA. 5. The pentose phosphate pathway and malic enzyme.

Structure: core of triacylglycerol and cholesteryl esters; surface of phospholipids, free cholesterol and apolipoproteins. Classes (from lowest to highest density) - Chylomicrons: from the intestine, carry dietary triacylglycerol (apoB-48, apoC-II, apoE). - VLDL: from the liver, carry endogenous triacylglycerol (apoB-100). - IDL: intermediate remnant. - LDL: main carrier of cholesterol to tissues (apoB-100). - HDL: carries cholesterol from tissues to the liver (apoA-I). Apolipoprotein functions: apoC-II activates lipoprotein lipase; apoB-100 binds the LDL receptor; apoA-I activates LCAT.

Exogenous: chylomicrons carry dietary fat through lymph into blood. Lipoprotein lipase (on capillary endothelium, activated by apoC-II) releases fatty acids to muscle and fat; the remnant is taken up by the liver. Endogenous: the liver secretes VLDL, lipoprotein lipase turns it into IDL and then LDL. LDL is taken up by LDL receptors on cells (receptor-mediated endocytosis), which downregulate cholesterol synthesis. Reverse transport: nascent HDL takes cholesterol from cells, LCAT esterifies it, and the cholesterol returns to the liver. Clinical tip: familial hypercholesterolaemia (LDL receptor defect) causes very high LDL, xanthomas and early coronary disease. High LDL and low HDL increase atherosclerosis risk.

1. Which lipoprotein carries dietary triacylglycerol? 2. What activates lipoprotein lipase? 3. What is the source of LDL? 4. What is the role of HDL? 5. What defect causes familial hypercholesterolaemia?

1. Chylomicrons. 2. Apolipoprotein C-II. 3. Metabolism of VLDL through IDL. 4. Reverse cholesterol transport to the liver. 5. A defective LDL receptor.

Fed state (up to 4 hours after eating): insulin high. The liver takes up glucose (GLUT2), stores glycogen, makes fatty acids and triacylglycerol (exported as VLDL) and uses amino acids. Post-absorptive and fasting: glucagon high. The liver breaks down glycogen for the first 12-24 hours, then relies on gluconeogenesis (from lactate, glycerol, amino acids). Fatty acid oxidation increases and ketogenesis starts. The liver also makes urea, plasma proteins and bile, and detoxifies drugs. Hormones: insulin is anabolic; glucagon, adrenaline, cortisol and growth hormone are counter-regulatory.

Fed state: insulin increases glucose uptake (GLUT4), promotes lipoprotein lipase, fatty acid uptake and triacylglycerol synthesis (glycerol 3-phosphate from glucose), and inhibits hormone-sensitive lipase. Fasting: glucagon and adrenaline activate hormone-sensitive lipase, releasing fatty acids and glycerol (glycerol is used by the liver for gluconeogenesis). White adipose tissue stores energy and secretes hormones (leptin, adiponectin); brown adipose tissue uses thermogenin (UCP1) to make heat. Insulin resistance in obesity impairs suppression of lipolysis and raises circulating fatty acids.

Exocrine: acinar cells secrete digestive enzymes (amylase, lipase, proteases as zymogens) and duct cells secrete bicarbonate. Endocrine: beta cells secrete insulin and alpha cells secrete glucagon. Glucose is the main trigger of insulin release: GLUT2 uptake, glucokinase as sensor, rise in ATP, closure of the K-ATP channel, calcium entry. Amino acids and incretins also stimulate insulin, and fatty acids modulate it. The two hormones keep the blood glucose within about 4-6 mmol/L.

Skeletal muscle, fed state: glucose uptake by GLUT4 (insulin), glycogen synthesis, protein synthesis. At rest fatty acids are the main fuel. Exercise: creatine phosphate gives immediate ATP; anaerobic glycolysis gives lactate (sent to the liver, Cori cycle); aerobic oxidation of glucose and fatty acids sustains prolonged work. Fasting: muscle uses fatty acids and ketones, spares glucose, and releases amino acids (alanine and glutamine) from proteolysis. Cardiac muscle is entirely aerobic and rich in mitochondria; it prefers fatty acids (60-70 percent) but takes up lactate and glucose, and can use ketones. It has little glycogen and no anaerobic reserve.

Brain: about 2 percent of body weight and 20 percent of resting energy use. In the fed state, glucose is nearly the only fuel (GLUT1 at the barrier and GLUT3 in neurons). Astrocytes release lactate that neurons use (lactate shuttle). In early fasting the brain still uses glucose from gluconeogenesis; after 2-3 days it uses ketone bodies for a large fraction of its energy, reducing protein breakdown. Fatty acids cannot cross the barrier. Red blood cells: no mitochondria, so they depend exclusively on glycolysis and produce lactate. The Rapoport-Luebering shunt makes 2,3-bisphosphoglycerate, which lowers haemoglobin's oxygen affinity. The pentose phosphate pathway supplies NADPH to keep glutathione reduced. Membrane lipid repair and nitric oxide synthase are further functions. Clinical tip: pyruvate kinase and G6PD deficiencies cause haemolytic anaemia.

Mammary gland: during lactation it draws large amounts of glucose (for lactose synthesis and as fuel), amino acids (milk proteins) and fatty acids (milk fat), and makes fatty acids de novo. It is regulated by prolactin and oxytocin, and lactose synthesis needs alpha-lactalbumin as a modifier of galactosyltransferase. In fasting the maternal body diverts substrate to the gland. Cornea: transparent and avascular, so its glucose comes from the aqueous humour and its oxygen from the air and tears. It uses glycolysis and the pentose phosphate pathway and produces lactate when oxygen is scarce, for example under a closed eyelid or a contact lens, which can cause corneal swelling.

1. Which hormone dominates in the fed state and which in fasting? 2. How does adipose tissue respond to insulin? 3. Why does the brain use ketone bodies in prolonged fasting? 4. Why do red cells produce lactate? 5. What is the role of the Cori cycle in exercise?

1. Insulin in the fed state, glucagon in fasting. 2. It takes up glucose, stimulates lipoprotein lipase and triacylglycerol synthesis and inhibits lipolysis. 3. To spare glucose and reduce protein breakdown; fatty acids cannot cross the blood-brain barrier. 4. They have no mitochondria and rely only on anaerobic glycolysis. 5. It returns lactate from muscle to the liver for gluconeogenesis, then glucose goes back to muscle.