Glycolysis, Gluconeogenesis, Citric Acid Cycle, and Oxidative Phosphorylation

Vocabulary flashcards covering key metabolic concepts, enzymes, regulators, pathways, and clinical conditions related to glycolysis, gluconeogenesis, the citric acid cycle, and oxidative phosphorylation.

Glycolysis

A central cytosolic pathway present in all tissues that breaks down 6-carbon monosaccharides into 2 pyruvate molecules, producing a net 2 ATP and 2 NADH.

Hexokinase

A glycolytic enzyme present in all tissues that phosphorylates glucose to glucose-6-phosphate, possessing high affinity (low $K_m$) and low capacity (low $V_{max}$) for glucose.

Glucokinase

Hexokinase isoenzyme IV specific to liver and pancreatic $\beta$ cells that operates at high glucose levels, having low affinity (high $K_m$) and high capacity (high $V_{max}$) to trap glucose in liver cells.

Phosphofructokinase-1 (PFK-1)

The rate-limiting, irreversible enzyme of glycolysis that converts fructose-6-phosphate to fructose-1,6-bisphosphate; inhibited by ATP and citrate, and activated by AMP and fructose-2,6-bisphosphate.

Pyruvate kinase (PK)

The irreversible enzyme in the energy generation phase of glycolysis that converts phosphoenolpyruvate (PEP) to pyruvate; inactivated by PKA-mediated phosphorylation and activated by PP1 dephosphorylation.

Lactate dehydrogenase (LDH)

A cytosolic enzyme that reduces pyruvate to lactate in anaerobic conditions while regenerating $\text{NAD}^+$ required for continued glycolysis.

Pyruvate kinase deficiency

A rare glycolytic disorder causing premature destruction of red blood cells and chronic hemolytic anemia due to insufficient ATP production needed to maintain RBC shape and lifespan.

Gluconeogenesis

An energy-consuming pathway occurring mainly in the liver during prolonged fasting that generates glucose from non-carbohydrate precursors (glycerol, lactate, amino acids), consuming 4 ATP, 2 GTP, and 2 NADH per glucose.

Cori Cycle

The metabolic pathway in which lactate generated by anaerobic tissues (such as exercising muscle and RBCs) is transported to the liver, converted to glucose via gluconeogenesis, and returned to circulation.

Pyruvate carboxylase

A mitochondrial bypass enzyme of gluconeogenesis that converts pyruvate to oxaloacetate (OAA).

PEP carboxykinase (PEPCK)

A cytosolic bypass enzyme of gluconeogenesis that converts oxaloacetate (OAA) into phosphoenolpyruvate (PEP); its transcription is increased by glucagon and cortisol.

Fructose 1,6-bisphosphatase

A gluconeogenic bypass enzyme found only in the liver and kidneys that converts fructose-1,6-bisphosphate to fructose-6-phosphate; activated by low fructose-2,6-bisphosphate levels.

Glucose 6-phosphatase

The final bypass enzyme of gluconeogenesis located in the liver and kidneys that hydrolyzes glucose-6-phosphate to free glucose for release into the blood.

Fructose 2,6-bisphosphate (F2,6BP)

A potent allosteric effector that activates PFK-1 (promoting glycolysis) and inhibits fructose 1,6-bisphosphatase (inhibiting gluconeogenesis); its levels are elevated by insulin and decreased by glucagon.

Mitochondrion

The double-membraned organelle where the citric acid cycle and oxidative phosphorylation take place; contains a permeable outer membrane, an impermeable inner membrane with cristae, and an enzyme-rich matrix.

Pyruvate dehydrogenase (PDH) complex

A mitochondrial matrix complex (comprising E1, E2, E3) that irreversibly converts pyruvate to acetyl CoA, producing 1 NADH and 1 $\text{CO}_2$ per pyruvate.

Thiamine pyrophosphate (TPP)

The active coenzyme derived from thiamine (vitamin B1) required by enzyme E1 (pyruvate decarboxylase) of the pyruvate dehydrogenase complex.

Arsenic poisoning

Inhibition of enzymatic complexes requiring lipoic acid, specifically blocking lipoic acid from acting as a coenzyme for E2 in the pyruvate dehydrogenase complex.

Citric Acid Cycle

A metabolic pathway in the mitochondrial matrix that completely oxidizes acetyl CoA, generating 3 NADH, 1 $\text{FADH}_2$, 1 GTP, and 2 $\text{CO}_2$ per round.

Citrate synthase

The initiating enzyme of the citric acid cycle that catalyzes the irreversible condensation of acetyl CoA and oxaloacetate to form citrate.

Isocitrate dehydrogenase

A rate-limiting enzyme of the citric acid cycle that converts isocitrate to $\alpha$-ketoglutarate, producing 1 NADH and 1 $\text{CO}_2$; activated by ADP and $\text{Ca}^{2+}$.

Succinate dehydrogenase

The only citric acid cycle enzyme embedded in the inner mitochondrial membrane (acting as Complex II in the ETC) that converts succinate to fumarate and generates 1 $\text{FADH}_2$.

Electron Transport Chain (ETC)

A series of four inner mitochondrial membrane complexes and mobile carriers (CoQ, Cytochrome C) that transfer electrons to oxygen, creating a proton gradient across the inner membrane.

Coenzyme Q (CoQ)

A mobile electron carrier in the inner mitochondrial membrane that accepts electrons from Complexes I and II and delivers them to Complex III.

Cytochrome C

A mobile electron carrier that transfers electrons from Complex III to Complex IV in the electron transport chain.

Oxygen

The final electron acceptor of the electron transport chain at Complex IV, combining with electrons and protons to form water.

ATP synthase

Complex V of oxidative phosphorylation, consisting of an $F_0$ inner membrane proton channel and an $F_1$ catalytic matrix domain that synthesizes ATP from ADP and $\text{P}_i$ driven by proton flow.

Oligomycin

A drug that inhibits oxidative phosphorylation by binding to the $F_0$ domain of ATP synthase and closing the proton channel.

Thermogenin (UCP1)

A natural uncoupling protein in brown adipocytes that allows protons to re-enter the mitochondrial matrix without driving ATP synthesis, releasing energy as heat.

Ophthalmoplegia

A symptom of mitochondrial dysfunction characterized by the inability to move the eyes normally (requiring turning of the head), caused by weakening of the ATP-demanding eye muscles.

Ptosis

Eyelid drooping caused by insufficient ATP production in the eyelid muscles, often associated with mitochondrial disorders.