Carbohydrate Metabolism & Glycolysis

Carbohydrates are the primary fuel source for most cells, and the metabolic pathways that process glucose are among the most conserved in biology. Understanding glucose catabolism from glycolysis through oxidative phosphorylation is fundamental to comprehending energy metabolism in health and disease.

Glucose Structure and Properties

Glucose (C6H12O6) is an aldohexose that exists predominantly in its cyclic pyranose form in solution. The alpha and beta anomers differ in the orientation of the hydroxyl group at C1 (the anomeric carbon): alpha-D-glucose has the C1-OH axial (same side as the ring oxygen reference), while beta-D-glucose has C1-OH equatorial. Glucose transporters (GLUTs) are facilitative transporters that move glucose down its concentration gradient. GLUT1 is ubiquitous; GLUT2 (high Km, high capacity) is found in liver, pancreatic beta cells, and intestine; GLUT4 is insulin-sensitive and mediates glucose uptake in muscle and adipose tissue.

Glycolysis: The Ten Steps

Glycolysis occurs in the cytoplasm and converts one molecule of glucose into two molecules of pyruvate, yielding a net of 2 ATP and 2 NADH per glucose. The pathway has three phases:

Investment phase (steps 1-3): Glucose is phosphorylated to glucose-6-phosphate by hexokinase (or glucokinase in liver/pancreas), consuming 1 ATP. Phosphoglucose isomerase converts G6P to fructose-6-phosphate. Phosphofructokinase-1 (PFK-1) phosphorylates F6P to fructose-1,6-bisphosphate, consuming a second ATP -- this is the major committed and rate-limiting step.

Cleavage phase (steps 4-5): Aldolase cleaves F1,6-BP into dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). Triose phosphate isomerase interconverts DHAP and G3P, so both trioses enter the payoff phase.

Payoff phase (steps 6-10, x2 for each triose): G3P is oxidised and phosphorylated by glyceraldehyde-3-phosphate dehydrogenase to 1,3-bisphosphoglycerate, producing 2 NADH. Phosphoglycerate kinase transfers the high-energy phosphate to ADP (substrate-level phosphorylation, 2 ATP). Phosphoglycerate mutase and enolase produce phosphoenolpyruvate (PEP). Pyruvate kinase transfers the phosphate from PEP to ADP, yielding 2 more ATP and 2 pyruvate.

Net per glucose: -2 ATP (investment) + 4 ATP (payoff) = 2 net ATP; 2 NADH.

Key Regulatory Enzymes

PFK-1 is the primary regulatory point. It is allosterically inhibited by ATP and citrate (signals of energy sufficiency) and activated by AMP, ADP, and fructose-2,6-bisphosphate (F2,6-BP). F2,6-BP is the most potent activator; its level is controlled by the bifunctional enzyme PFK-2/FBPase-2, which is regulated by insulin (activates kinase domain) and glucagon/epinephrine (via PKA, activates phosphatase domain). Hexokinase is inhibited by its product G6P (product inhibition), whereas glucokinase (hexokinase IV) is not, allowing the liver to act as a glucose buffer. Pyruvate kinase is inhibited by ATP and alanine and activated by F1,6-BP (feedforward activation).

Fate of Pyruvate: Aerobic vs Anaerobic

Under aerobic conditions, pyruvate enters the mitochondrial matrix and is oxidatively decarboxylated to acetyl-CoA by the pyruvate dehydrogenase complex (PDC) -- a multienzyme complex using cofactors thiamine pyrophosphate (TPP), lipoic acid, FAD, NAD+, and CoA. This reaction also yields one NADH and one CO2 per pyruvate. PDC is inhibited by its products (acetyl-CoA and NADH) and by phosphorylation via PDH kinase (activated by ATP, NADH, acetyl-CoA); it is activated by PDH phosphatase (stimulated by Ca2+ and insulin). Under anaerobic conditions (or in red blood cells, which lack mitochondria), pyruvate is reduced to lactate by lactate dehydrogenase (LDH), regenerating NAD+ to allow glycolysis to continue. This is the basis of the Cori cycle: lactate produced in muscle travels to the liver, where it is reconverted to glucose via gluconeogenesis.

The TCA Cycle

Acetyl-CoA (2 carbons) condenses with oxaloacetate (4 carbons) to form citrate (6 carbons) -- catalysed by citrate synthase. The cycle then proceeds through isocitrate, alpha-ketoglutarate (with loss of CO2 and production of NADH), succinyl-CoA (with another CO2 and NADH), succinate (with production of GTP by substrate-level phosphorylation and FADH2), fumarate, malate, and back to oxaloacetate (with a final NADH). Per acetyl-CoA: 3 NADH, 1 FADH2, 1 GTP, 2 CO2. Anaplerosis refers to reactions that replenish TCA intermediates: pyruvate carboxylase converts pyruvate to oxaloacetate (important in gluconeogenesis and when biosynthetic intermediates are withdrawn); transamination reactions interconvert amino acids and TCA intermediates (e.g., aspartate <-> oxaloacetate, glutamate <-> alpha-ketoglutarate).

Oxidative Phosphorylation

The electron transport chain (ETC) is located in the inner mitochondrial membrane. NADH donates electrons to Complex I (NADH:ubiquinone oxidoreductase); FADH2 donates to Complex II (succinate dehydrogenase). Electrons pass via ubiquinone (coenzyme Q) to Complex III (cytochrome bc1) and via cytochrome c to Complex IV (cytochrome c oxidase), which reduces O2 to water. Complexes I, III, and IV pump protons from the matrix to the intermembrane space, creating an electrochemical gradient (proton-motive force). ATP synthase (Complex V) harnesses this gradient: protons flow back through the Fo subunit, driving rotation of the F1 subunit and phosphorylation of ADP to ATP (chemiosmosis, as proposed by Peter Mitchell). The P/O ratio (ATP produced per oxygen atom reduced) is approximately 2.5 for NADH and 1.5 for FADH2 in modern estimates. Complete oxidation of one glucose yields approximately 30-32 ATP.

Gluconeogenesis

Gluconeogenesis synthesises glucose from non-carbohydrate precursors -- lactate, glycerol, and glucogenic amino acids (especially alanine and glutamine). It occurs mainly in the liver (and to a lesser extent the kidney cortex). Most glycolytic reactions are reversible and are simply run in reverse, but three irreversible steps of glycolysis require unique gluconeogenic enzymes: pyruvate carboxylase (pyruvate -> oxaloacetate, mitochondria) + PEPCK (OAA -> PEP, bypassing pyruvate kinase); fructose-1,6-bisphosphatase (F1,6-BP -> F6P, bypassing PFK-1); glucose-6-phosphatase (G6P -> glucose, in ER, bypassing hexokinase). Gluconeogenesis is energetically expensive (6 ATP equivalents per glucose) and is stimulated by glucagon and cortisol, inhibited by insulin.

Glycogen Metabolism

Glycogen is a highly branched polymer of glucose (alpha-1,4 linkages with alpha-1,6 branches every 8-12 residues), stored in liver (glycogen buffer for blood glucose) and muscle (local fuel). Glycogen synthesis: glucose is activated to UDP-glucose (by UDP-glucose pyrophosphorylase); glycogen synthase extends chains via alpha-1,4 bonds; branching enzyme creates alpha-1,6 branches. Glycogen breakdown: glycogen phosphorylase cleaves glucose-1-phosphate from the non-reducing ends (requires PLP cofactor); debranching enzyme removes branch points. Regulation is hormonal and allosteric: glucagon/epinephrine activate phosphorylase (via PKA -> phosphorylase kinase) and inhibit synthase; insulin reverses this by activating phosphoprotein phosphatase 1.

Metabolic Diseases

G6PD (glucose-6-phosphate dehydrogenase) deficiency impairs the pentose phosphate pathway, reducing NADPH production and leaving red blood cells vulnerable to oxidative haemolysis -- triggered by certain drugs (primaquine, dapsone), infections, or fava beans. Glycogen storage diseases (GSDs) result from enzyme deficiencies in glycogen metabolism: Pompe disease (GSD II, acid alpha-glucosidase deficiency) causes lysosomal glycogen accumulation with cardiomyopathy; McArdle disease (GSD V, muscle phosphorylase deficiency) causes exercise intolerance. Lactic acidosis arises when pyruvate cannot enter the TCA cycle (PDC deficiency, thiamine deficiency/Wernicke encephalopathy) or when anaerobic metabolism is overwhelmed (shock, metformin toxicity). Type 2 diabetes represents dysregulation of glucose homeostasis with insulin resistance in peripheral tissues and inappropriate hepatic gluconeogenesis.