Metabolism, Bioenergetics, Hormones, Vitamins and Minerals

Every muscle contraction you palpate, every nerve impulse you test and every healing ligament you treat runs on the chemistry in this chapter. Part I questions on metabolism reward students who know where each pathway happens, which enzyme commits it, what controls it and what a missing enzyme or vitamin does to a patient.

Section 1Biochemical energetics: from glucose to ATP

Free energy and why reactions go

The direction of any biochemical reaction is set by its change in Gibbs free energy (ΔG). A negative ΔG means the reaction is exergonic and can proceed spontaneously; a positive ΔG means it is endergonic and needs an energy input. The standard free energy change (ΔG°′) describes the reaction at 1 M concentrations and pH 7, but the actual ΔG inside a cell depends on the ratio of products to reactants: ΔG = ΔG°′ + RT ln([products]/[reactants]). This is why a reaction with a mildly positive standard value can still run forward in a cell when its product is quickly removed. Enzymes never change ΔG or the equilibrium constant; they lower the activation energy and so change only the rate.

Cells couple unfavorable reactions to favorable ones, and the most common coupling agent is adenosine triphosphate (ATP). Hydrolysis of ATP to ADP and inorganic phosphate releases about 7.3 kcal/mol under standard conditions (closer to 12 kcal/mol under cellular conditions). The energy lies in the two phosphoanhydride bonds between the beta and gamma phosphates; their hydrolysis relieves electrostatic repulsion among negative charges and yields products stabilized by resonance and hydration. Other high energy compounds include phosphoenolpyruvate (the highest phosphoryl transfer potential in glycolysis), 1,3-bisphosphoglycerate, creatine phosphate (the muscle buffer that regenerates ATP for the first seconds of intense exercise) and the thioester acetyl-CoA.

Redox carriers

Most ATP is made indirectly. Fuels are oxidized, and their electrons are captured by carrier coenzymes that later deliver them to the electron transport chain. NAD+ (from niacin, vitamin B3) accepts a hydride ion and becomes NADH; it serves the catabolic dehydrogenases of glycolysis, pyruvate dehydrogenase and the citric acid cycle. FAD (from riboflavin, B2) accepts two hydrogen atoms to become FADH2 and is usually bound tightly to its enzyme, as in succinate dehydrogenase. NADPH is the reducing currency of anabolism: fatty acid and cholesterol synthesis, the respiratory burst of neutrophils, regeneration of reduced glutathione and the cytochrome P450 system. The cell keeps the NAD+/NADH ratio high (favoring oxidation) and the NADP+/NADPH ratio low (favoring reduction), which is how one cell can run breakdown and synthesis with separate pools.

Glycolysis

Glycolysis occurs in the cytosol of every cell and converts one glucose into two pyruvate, with a net gain of 2 ATP and 2 NADH. It has an investment phase (two ATP spent) and a payoff phase (four ATP made by substrate level phosphorylation). Three reactions are essentially irreversible and are the points of control.

Hexokinase has a low Km (high affinity) and low Vmax, so tissues such as brain and red cells trap glucose even when it is scarce. Glucokinase has a high Km and high Vmax, so the liver and beta cell respond in proportion to blood glucose after a meal, which makes glucokinase the beta cell's glucose sensor. A heterozygous glucokinase mutation causes a mild form of maturity onset diabetes of the young (MODY 2).

Fructose-2,6-bisphosphate is the hormonal switch. It is made by the kinase domain of the bifunctional enzyme PFK-2/FBPase-2. Insulin dephosphorylates this enzyme, activating the kinase and raising fructose-2,6-bisphosphate, which turns glycolysis on and gluconeogenesis off. Glucagon, through cAMP and protein kinase A, phosphorylates it and does the opposite in the liver.

The fate of pyruvate depends on oxygen and mitochondria. With adequate oxygen, pyruvate enters the mitochondrion. Under anaerobic conditions, or in cells without mitochondria such as red blood cells, lactate dehydrogenase reduces pyruvate to lactate and regenerates NAD+, which keeps glycolysis running at glyceraldehyde-3-phosphate dehydrogenase. Lactate travels to the liver and becomes glucose again (the Cori cycle). Red cells also have a side path, the Rapoport Luebering shunt, that makes 2,3-bisphosphoglycerate, which lowers hemoglobin's oxygen affinity and shifts the dissociation curve to the right. Pyruvate kinase deficiency, the second most common red cell enzyme defect after G6PD deficiency, causes a chronic hemolytic anemia because the cells cannot make enough ATP to maintain their membrane pumps.

Pyruvate dehydrogenase: the bridge

The pyruvate dehydrogenase complex (PDH) in the mitochondrial matrix converts pyruvate to acetyl-CoA, releasing CO2 and making NADH. The reaction is irreversible, which is why acetyl-CoA can never be turned back into glucose in humans and why even chain fatty acids are not glucogenic. PDH needs five cofactors, remembered by the phrase "Tender Loving Care For Nancy": thiamine pyrophosphate (B1), lipoic acid, coenzyme A (from pantothenate, B5), FAD (B2) and NAD+ (B3). The same set serves alpha-ketoglutarate dehydrogenase in the citric acid cycle and the branched chain alpha-ketoacid dehydrogenase that is defective in maple syrup urine disease.

PDH is activated by insulin, by high ADP and pyruvate, and in muscle by calcium; it is inhibited by its products acetyl-CoA and NADH and by ATP. These inhibitors act through a PDH kinase that phosphorylates and inactivates the complex; PDH phosphatase reactivates it. Arsenite binds lipoic acid and inhibits both PDH and alpha-ketoglutarate dehydrogenase. Thiamine deficiency impairs PDH, which is why giving glucose to a malnourished alcoholic patient before thiamine can precipitate Wernicke encephalopathy: the extra glucose drives flux into a blocked complex. PDH deficiency, usually X linked, causes congenital lactic acidosis with neurologic problems; treatment uses a ketogenic diet high in fat and in the purely ketogenic amino acids lysine and leucine, because these feed acetyl-CoA without passing through PDH.

The citric acid cycle

The citric acid cycle (Krebs or tricarboxylic acid cycle) runs in the mitochondrial matrix. Acetyl-CoA (2 carbons) condenses with oxaloacetate (4 carbons) to form citrate (6 carbons) by citrate synthase. The cycle then proceeds through isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate and malate back to oxaloacetate. Two carbons leave as CO2 per turn. A useful mnemonic for the intermediates is "Citrate Is Krebs' Starting Substrate For Making Oxaloacetate."

Each turn yields 3 NADH, 1 FADH2 and 1 GTP (made by succinyl-CoA synthetase, also called succinate thiokinase). Because each glucose gives two acetyl-CoA, the cycle turns twice per glucose. Isocitrate dehydrogenase is the rate limiting enzyme; it is activated by ADP and calcium and inhibited by ATP and NADH. Alpha-ketoglutarate dehydrogenase is inhibited by its products succinyl-CoA and NADH. Succinate dehydrogenase is the only enzyme of the cycle embedded in the inner mitochondrial membrane and doubles as Complex II of the electron transport chain.

The cycle is amphibolic: it serves both catabolism and synthesis. Citrate leaves the mitochondrion to supply acetyl-CoA for fatty acid synthesis; alpha-ketoglutarate and oxaloacetate are converted to glutamate and aspartate; succinyl-CoA is the starting point for heme synthesis. Intermediates drawn off must be replaced by anaplerotic reactions, the most important being pyruvate carboxylase (pyruvate to oxaloacetate, biotin dependent, activated by acetyl-CoA).

Electron transport and oxidative phosphorylation

The electron transport chain sits in the inner mitochondrial membrane. NADH donates electrons to Complex I (NADH dehydrogenase); FADH2 from succinate enters at Complex II. Both pass electrons to coenzyme Q (ubiquinone), then to Complex III (cytochrome bc1), cytochrome c and finally Complex IV (cytochrome c oxidase), which reduces oxygen to water. Complexes I, III and IV pump protons from the matrix into the intermembrane space, building an electrochemical proton gradient (the proton motive force). This is Mitchell's chemiosmotic theory. Protons flow back through Complex V (ATP synthase), and the rotation of its F0 portion drives ATP formation in the F1 head.

Because FADH2 bypasses Complex I, it pumps fewer protons. The current accepted yields are about 2.5 ATP per NADH and 1.5 ATP per FADH2, giving 30 to 32 ATP per glucose aerobically (the older textbook figures were 3, 2 and 36 to 38). The range depends on which shuttle moves cytosolic NADH into the mitochondrion: the malate aspartate shuttle (liver, heart, kidney) preserves the full NADH value, while the glycerol-3-phosphate shuttle (skeletal muscle, brain) hands electrons to FAD and loses about one ATP per NADH. Anaerobic glycolysis gives only 2 ATP per glucose.

Uncouplers separate electron flow from ATP synthesis. Electron transport and oxygen consumption accelerate because the gradient is constantly dissipated, but the energy appears as heat. Brown adipose tissue uses the natural uncoupling protein thermogenin for nonshivering thermogenesis in newborns. Salicylate poisoning produces hyperthermia partly by uncoupling. By contrast, inhibitors of the chain stop oxygen consumption, and ATP synthase inhibitors stop it indirectly because the gradient cannot be relieved.

Section 2Carbohydrate metabolism and its control

Gluconeogenesis

Gluconeogenesis builds glucose from noncarbohydrate precursors: lactate (Cori cycle), glycerol (from triglyceride breakdown), glucogenic amino acids (chiefly alanine through the glucose alanine cycle, and glutamine) and propionyl-CoA from odd chain fatty acids. It occurs mainly in the liver (about 90 percent) and the renal cortex, with the kidney's share rising in prolonged fasting. Muscle cannot release free glucose because it lacks glucose-6-phosphatase.

The pathway reverses glycolysis but must bypass its three irreversible steps with four unique enzymes.

Making one glucose from two pyruvate costs 6 high energy phosphates (4 ATP and 2 GTP) plus 2 NADH. The ATP for this comes from beta oxidation of fatty acids, which also supplies the acetyl-CoA that activates pyruvate carboxylase. This explains why disorders of fatty acid oxidation cause hypoketotic hypoglycemia: the liver cannot generate the energy or the signal for gluconeogenesis. Ethanol inhibits gluconeogenesis by raising the NADH/NAD+ ratio, which pushes pyruvate to lactate and oxaloacetate to malate, draining the substrates; a fasting binge drinker can therefore become severely hypoglycemic.

Glycogen synthesis and breakdown

Glycogen is a branched glucose polymer with alpha-1,4 linkages in its chains and alpha-1,6 linkages at branch points roughly every 8 to 12 residues. It is stored mainly in liver, which uses it to maintain blood glucose for roughly the first 12 to 24 hours of fasting, and in skeletal muscle, which uses it only for its own contraction.

Synthesis (glycogenesis): glucose-6-phosphate is converted to glucose-1-phosphate by phosphoglucomutase, then activated to UDP-glucose by UDP-glucose pyrophosphorylase. Glycogen synthase, the rate limiting enzyme, adds glucose in alpha-1,4 linkage to a primer on the protein glycogenin. The branching enzyme moves a segment of six to eight residues to an alpha-1,6 position.

Breakdown (glycogenolysis): glycogen phosphorylase, the rate limiting enzyme, cleaves alpha-1,4 bonds with inorganic phosphate (phosphorolysis) to release glucose-1-phosphate. It requires pyridoxal phosphate (B6) and stops four residues from a branch point. The debranching enzyme has two activities: a transferase that moves three of those residues to another chain, and an alpha-1,6 glucosidase that releases the last residue as free glucose. In liver, glucose-6-phosphatase then frees glucose for the blood; in muscle, glucose-6-phosphate enters glycolysis. A small amount of glycogen is also degraded in lysosomes by acid maltase (lysosomal alpha-1,4 glucosidase).

The pentose phosphate pathway

The pentose phosphate pathway (hexose monophosphate shunt) runs in the cytosol and produces no ATP. Its oxidative phase is irreversible: glucose-6-phosphate dehydrogenase (G6PD), the rate limiting enzyme, generates NADPH, and the phase ends with ribulose-5-phosphate and CO2. The nonoxidative phase is reversible and uses transketolase (which requires thiamine) to interconvert sugars, producing ribose-5-phosphate for nucleotides or returning carbons to glycolytic intermediates. The pathway is most active in liver, adrenal cortex, lactating mammary gland, gonads and red cells, which are all tissues that synthesize fatty acids or steroids or must fight oxidative stress.

Red cells depend entirely on this pathway for NADPH, which keeps glutathione reduced so it can neutralize hydrogen peroxide. In G6PD deficiency (X linked recessive, the most common human enzyme deficiency, protective against malaria), oxidant stress from infection, fava beans, sulfonamides, primaquine, dapsone or nitrofurantoin causes acute hemolysis. Denatured hemoglobin precipitates as Heinz bodies, and splenic macrophages remove them, leaving bite cells. Erythrocyte transketolase activity is a classic test of thiamine status.

Fructose and galactose

Fructose is phosphorylated in the liver by fructokinase to fructose-1-phosphate, which aldolase B splits into dihydroxyacetone phosphate and glyceraldehyde. This route skips PFK-1, so fructose enters glycolysis without its main control point, which partly explains why high fructose intake promotes hepatic fat synthesis. Galactose, from lactose, is phosphorylated by galactokinase and then converted by galactose-1-phosphate uridyltransferase to UDP-galactose and on to glucose. The polyol pathway converts glucose to sorbitol by aldose reductase and then to fructose by sorbitol dehydrogenase; tissues with little sorbitol dehydrogenase (lens, retina, Schwann cells, kidney) accumulate sorbitol in hyperglycemia, contributing to diabetic cataracts and neuropathy.

Glycogen storage diseases

The glycogen storage diseases follow a pattern: when the liver enzyme is missing, the patient has fasting hypoglycemia and hepatomegaly; when the muscle enzyme is missing, the patient has exercise intolerance, cramps and myoglobinuria. Almost all are autosomal recessive. The mnemonic "Very Poor Carbohydrate Metabolism" lists types I to V in order: von Gierke, Pompe, Cori and McArdle (type IV, Andersen, is the branching enzyme defect).

Hormonal integration of the fed and fasting states

After a meal, insulin dominates. It promotes glucose uptake through GLUT4 in muscle and adipose tissue, activates glycolysis, glycogen synthase, PDH, acetyl-CoA carboxylase and lipoprotein lipase, and suppresses gluconeogenesis, glycogenolysis and lipolysis. Insulin acts largely through dephosphorylation. In fasting, glucagon (liver) and epinephrine (liver, muscle and fat) raise cAMP and phosphorylate the same enzymes, reversing each switch. Cortisol and growth hormone add slower support by inducing gluconeogenic enzymes and antagonizing insulin. Glucose transporters matter too: GLUT1 (red cells, brain, insulin independent), GLUT2 (liver, beta cells, kidney, intestine, bidirectional and low affinity), GLUT3 (neurons, high affinity), GLUT4 (muscle and fat, insulin dependent) and GLUT5 (fructose, intestine and sperm).

The fasting timeline follows predictable phases: glycogenolysis carries the first day, gluconeogenesis rises over the next days, and in prolonged starvation the brain adapts to use ketone bodies for up to two thirds of its energy, which spares muscle protein. Red cells never adapt, because they lack mitochondria, so they always need glucose.

Section 3Lipid and amino acid metabolism

Beta oxidation

Triglycerides in adipose tissue are hydrolyzed by hormone sensitive lipase (activated by epinephrine and glucagon, inhibited by insulin) and adipose triglyceride lipase. Free fatty acids travel on albumin; glycerol goes to the liver for gluconeogenesis. Inside the target cell, a fatty acid is activated to fatty acyl-CoA in the cytosol. Long chain acyl groups cannot cross the inner mitochondrial membrane directly, so the carnitine shuttle carries them: carnitine palmitoyltransferase I (CPT-I) on the outer membrane is the rate limiting step and is inhibited by malonyl-CoA, the first product of fatty acid synthesis. That single inhibition prevents a cell from making and burning fat at the same time.

In the matrix, each round of beta oxidation has four steps (oxidation by acyl-CoA dehydrogenase producing FADH2, hydration, oxidation producing NADH, and thiolytic cleavage) and removes two carbons as acetyl-CoA. Palmitate (16 carbons) passes through seven rounds, yielding 8 acetyl-CoA, 7 NADH and 7 FADH2, for a net of about 106 ATP. Odd chain fatty acids leave a final propionyl-CoA, which is carboxylated (biotin) and converted to methylmalonyl-CoA and then, by methylmalonyl-CoA mutase (which needs vitamin B12), to succinyl-CoA. Very long chain fatty acids begin oxidation in peroxisomes; their accumulation in X linked adrenoleukodystrophy damages adrenal cortex and white matter.

Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common fatty acid oxidation disorder. An infant who fasts during an illness develops vomiting, lethargy and hypoketotic hypoglycemia, sometimes mimicking sudden infant death. Carnitine deficiency and CPT-II deficiency cause muscle weakness, cramps and myoglobinuria with prolonged exercise or fasting, a contrast to McArdle disease where symptoms come early in exertion.

Ketogenesis

When beta oxidation in the liver produces more acetyl-CoA than the citric acid cycle can use, which happens in fasting, untreated type 1 diabetes and alcoholism (where oxaloacetate is diverted to gluconeogenesis or to malate), the liver mitochondria build ketone bodies. The rate limiting enzyme is HMG-CoA synthase (mitochondrial). HMG-CoA lyase releases acetoacetate, which is reduced to beta-hydroxybutyrate or decarboxylates spontaneously to acetone (the fruity breath). Ketones are exported to heart, muscle, kidney and, during prolonged fasting, brain, which convert them back to acetyl-CoA using succinyl-CoA:acetoacetate CoA transferase (thiophorase). The liver lacks thiophorase, so it cannot use the ketones it makes. Red cells cannot use them at all.

The nitroprusside urine dipstick detects acetoacetate (and acetone) but not beta-hydroxybutyrate. In diabetic and especially alcoholic ketoacidosis, the high NADH/NAD+ ratio pushes most ketone into beta-hydroxybutyrate, so the dipstick can underestimate the acidosis.

Fatty acid synthesis

Fatty acid synthesis occurs in the cytosol of liver, lactating breast and adipose tissue. Acetyl-CoA made in the mitochondrion exits as citrate (the citrate shuttle) and is regenerated by ATP citrate lyase. Acetyl-CoA carboxylase, the rate limiting enzyme, uses biotin and CO2 to make malonyl-CoA; it is activated by citrate and insulin and inhibited by glucagon, epinephrine (via AMP activated kinase and protein kinase A) and palmitoyl-CoA. The multifunctional fatty acid synthase then adds two carbon units, using NADPH from the pentose phosphate pathway and malic enzyme, until it releases palmitate (16:0). Elongation and desaturation occur in the endoplasmic reticulum, but humans cannot insert double bonds beyond carbon 9, so linoleic acid (omega 6) and alpha-linolenic acid (omega 3) are essential fatty acids. Arachidonic acid, made from linoleic acid, is the parent of the prostaglandins, thromboxanes and leukotrienes covered in pathology.

Cholesterol synthesis

All carbons of cholesterol come from acetyl-CoA. In the cytosol and smooth endoplasmic reticulum, acetyl-CoA forms HMG-CoA, and HMG-CoA reductase, the rate limiting enzyme, reduces it to mevalonate using NADPH. Mevalonate is converted through isoprenoid units (isopentenyl pyrophosphate, farnesyl pyrophosphate) to squalene, lanosterol and cholesterol. HMG-CoA reductase is induced by insulin and thyroid hormone, inhibited by glucagon, and suppressed by cellular cholesterol through the SREBP transcription system, which also controls LDL receptor number. Statins competitively inhibit the reductase; the fall in hepatocyte cholesterol up regulates LDL receptors and lowers plasma LDL. Because the isoprenoid branch also makes coenzyme Q and dolichol, statins lower CoQ10.

Cholesterol is the precursor of bile acids (rate limiting enzyme 7-alpha-hydroxylase), steroid hormones and vitamin D. Plasma lipids travel in lipoproteins: chylomicrons carry dietary triglyceride (apo B-48, C-II, E), VLDL carries hepatic triglyceride (apo B-100), LDL delivers cholesterol to tissues (apo B-100, binds the LDL receptor), and HDL returns cholesterol to the liver (apo A-I, which activates LCAT). Familial hypercholesterolemia (type IIa) is an autosomal dominant LDL receptor defect causing tendon xanthomas and early coronary disease.

Amino acid catabolism and the urea cycle

The first step in degrading most amino acids is transamination: an aminotransferase moves the alpha-amino group to alpha-ketoglutarate, forming glutamate. All aminotransferases need pyridoxal phosphate (B6). Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are the liver enzymes on a metabolic panel. Glutamate is then oxidatively deaminated by glutamate dehydrogenase, releasing free ammonia. Peripheral tissues ship nitrogen to the liver as glutamine (most tissues) and alanine (muscle, the glucose alanine cycle).

The urea cycle takes place in liver and spans two compartments. In the mitochondria, carbamoyl phosphate synthetase I (CPS-I), the rate limiting enzyme, combines ammonia, CO2 and 2 ATP; it requires the allosteric activator N-acetylglutamate, whose synthesis is stimulated by arginine. Ornithine transcarbamylase (OTC) joins carbamoyl phosphate to ornithine to form citrulline, which moves to the cytosol. Citrulline combines with aspartate (the source of urea's second nitrogen) to form argininosuccinate, which is cleaved to arginine and fumarate, and arginase splits arginine into urea and ornithine. Fumarate links the urea cycle to the citric acid cycle (the aspartate argininosuccinate shunt).

Urea cycle defects cause hyperammonemia: vomiting, lethargy, tremor (asterixis), cerebral edema and coma. Ammonia depletes alpha-ketoglutarate (shutting down the citric acid cycle in neurons) and raises glutamine, which swells astrocytes. OTC deficiency is the most common and the only X linked one; carbamoyl phosphate spills into pyrimidine synthesis, so orotic acid rises in the urine without megaloblastic anemia (which distinguishes it from hereditary orotic aciduria). CPS-I deficiency raises ammonia without orotic aciduria. Treatment restricts protein and uses lactulose (traps ammonium in the colon), rifaximin, and nitrogen scavengers such as sodium benzoate and phenylbutyrate. Acquired hyperammonemia in cirrhosis produces hepatic encephalopathy.

Products derived from amino acids

The only purely ketogenic amino acids are leucine and lysine. Isoleucine, phenylalanine, tyrosine, tryptophan and threonine are both ketogenic and glucogenic. The essential amino acids are phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine and lysine (arginine is conditionally essential in growth). Tyrosine becomes essential when phenylalanine hydroxylase is deficient.

Inborn errors of amino acid metabolism

Section 4Biochemistry of hormones

Three chemical classes

Hormones fall into three chemical classes, and the class predicts almost everything else about a hormone: how it is made and stored, how it travels in blood, how long it lasts and where its receptor is.

Peptide processing explains a useful lab fact: proinsulin is cleaved into insulin and C-peptide in equal amounts, so C-peptide measures endogenous insulin output. High insulin with low C-peptide suggests injected insulin; high insulin with high C-peptide suggests an insulinoma or sulfonylurea use. Likewise pro-opiomelanocortin (POMC) yields ACTH, MSH and beta-endorphin, which is why high ACTH states darken the skin.

Steroidogenesis

All steroid hormones start with cholesterol. The rate limiting step is the transport of cholesterol into the mitochondrion by StAR protein and its cleavage to pregnenolone by desmolase (cholesterol side chain cleavage enzyme, CYP11A1), which ACTH stimulates in the adrenal and LH stimulates in the gonads. Pregnenolone then branches into mineralocorticoids (zona glomerulosa), glucocorticoids (zona fasciculata) and androgens (zona reticularis). In peripheral tissues, 5-alpha-reductase converts testosterone to the more potent dihydrotestosterone, and aromatase converts androgens to estrogens.

Congenital adrenal hyperplasia results from a block in cortisol synthesis. Low cortisol removes negative feedback, ACTH rises and the adrenals enlarge, and precursors spill into whichever pathway remains open. 21-hydroxylase deficiency (over 90 percent of cases) lowers cortisol and aldosterone and raises androgens: salt wasting, hypotension, hyperkalemia, virilized female genitalia and elevated 17-hydroxyprogesterone. 11-beta-hydroxylase deficiency also virilizes but causes hypertension because 11-deoxycorticosterone, a mineralocorticoid, accumulates. 17-alpha-hydroxylase deficiency causes hypertension with sexual infantilism.

Receptors and second messengers

Water soluble hormones bind surface receptors and act through intracellular second messengers. The largest receptor family is the G protein coupled receptor (seven transmembrane domains). Hormone binding makes the alpha subunit exchange GDP for GTP, and the active alpha subunit regulates an effector until its intrinsic GTPase returns it to the GDP state.

Signal termination and toxins

Signals must turn off as precisely as they turn on. G proteins hydrolyze their own GTP; phosphodiesterases destroy cAMP and cGMP; phosphatases remove phosphates; receptors are desensitized by phosphorylation and internalization (down regulation). Toxins exploit these steps: cholera toxin ADP ribosylates Gs and locks it on, causing massive cAMP driven chloride and water secretion in the gut; pertussis toxin ADP ribosylates Gi and blocks it, which also raises cAMP. Sildenafil inhibits phosphodiesterase 5 and prolongs cGMP action.

Hormone receptors can also be the target of disease: Graves disease antibodies stimulate the TSH receptor, type 2 diabetes features post receptor insulin resistance, and androgen insensitivity syndrome is a defective intracellular androgen receptor in a 46,XY person with female external appearance. McCune Albright syndrome is a mosaic activating Gs mutation causing polyostotic fibrous dysplasia of bone, cafe au lait spots and precocious puberty.

Section 5Vitamins and minerals

Fat soluble vitamins

Vitamins A, D, E and K are absorbed with dietary fat in micelles, so fat malabsorption (cystic fibrosis, celiac disease, chronic pancreatitis, cholestasis, orlistat, bile acid sequestrants) lowers all four. They are stored in liver and adipose tissue, which allows toxicity (particularly A and D) and slows the onset of deficiency.

Water soluble vitamins

The water soluble vitamins (the B complex and C) are mostly not stored and are washed out in urine, so deficiency appears within weeks to months. B12 is the exception, with hepatic stores lasting several years. Most B vitamins act as coenzymes, which is how their deficiencies are best reasoned out: identify the reactions that need the coenzyme and the tissues that depend on those reactions.

Calcium and phosphorus

About 99 percent of body calcium and 85 percent of phosphorus sit in bone as hydroxyapatite. Plasma calcium (about 8.5 to 10.5 mg/dL) is roughly 45 percent ionized (the active form), 40 percent albumin bound and 15 percent complexed with anions. Alkalosis increases albumin binding and lowers ionized calcium, which is why hyperventilation produces tingling and carpopedal spasm. Hypoalbuminemia lowers total but not ionized calcium; the usual correction adds 0.8 mg/dL for each 1 g/dL fall in albumin below 4.

Three hormones control the balance. Parathyroid hormone, released when the calcium sensing receptor detects low calcium, raises calcium by stimulating osteoclast activity indirectly (through RANKL on osteoblasts), increasing distal tubular calcium reabsorption, decreasing proximal phosphate reabsorption (phosphaturia) and inducing renal 1-alpha-hydroxylase. Calcitriol (1,25-dihydroxyvitamin D) is made in sequence: 7-dehydrocholesterol in skin is converted by UVB light to cholecalciferol, the liver adds a 25-hydroxyl (25-hydroxyvitamin D is the best measure of stores), and the kidney adds the 1-hydroxyl. Calcitriol raises intestinal absorption of both calcium and phosphate. Calcitonin from thyroid C cells inhibits osteoclasts but plays a minor role in adults. FGF23 from osteocytes lowers phosphate by promoting its excretion and inhibiting calcitriol synthesis.

Iron, iodine, zinc and the other minerals

Iron studies are best read as a pattern. In iron deficiency, serum iron and ferritin fall while transferrin (TIBC) rises and saturation drops. In anemia of chronic disease, inflammatory cytokines (IL-6) raise hepcidin, trapping iron in macrophages: serum iron falls but ferritin is normal or high and TIBC falls. In hemochromatosis, iron, ferritin and saturation are high and TIBC low.

Words worth knowing