Metabolism: Glycolysis, the Citric Acid Cycle, and Oxidative Phosphorylation

Vocabulary practice flashcards covering cellular respiration, glycolysis, the citric acid cycle, and oxidative phosphorylation based on university lecture material.

Catabolism

The metabolic breakdown of complex molecules such as carbohydrates, fats, and proteins to release energy stored as $ATP$ and reduced electron carriers ($NADH$, $NADPH_2$, or $FADH_2$).

Anabolism

The biosynthetic process of 'building up' cellular components, which requires energy input via phosphoryl group transfer from $ATP$ and the reducing power of $NADH$ and $NADPH$.

Glycolysis

A 10-step metabolic pathway meaning 'sweet splitting' that extracts energy from one 6-carbon glucose molecule to yield two 3-carbon pyruvate molecules.

Preparatory Phase

The first phase of glycolysis (steps 1–5) where an investment of $2\,ATP$ is consumed to phosphorylate intermediates and cleave a hexose into two triose phosphates.

Pay-Off Phase

The second phase of glycolysis (steps 6–10) that produces a payout of $4\,ATP$, $2\,NADH$, and $2$ pyruvate molecules, yielding a net gain of $2\,ATP$ and $2\,NADH$.

Inorganic Phosphate ($P_i$)

A negatively charged chemical group added during phosphorylation that traps metabolites within the cell, provides recognition targets for enzymes, and redistributes electron density to drive reactions.

Hexokinase

A transferase enzyme that catalyzes step 1 of glycolysis, transferring a phosphate group from $ATP$ to carbon-6 of glucose to produce glucose 6-phosphate.

Phosphohexose Isomerase

An isomerase enzyme that catalyzes step 2 of glycolysis, rearranging glucose 6-phosphate into fructose 6-phosphate to present carbon-1 for subsequent phosphorylation.

Phosphofructokinase-1 (PFK-1)

A transferase enzyme catalyzing step 3 of glycolysis, which uses $1\,ATP$ to phosphorylate fructose 6-phosphate to fructose 1,6-bisphosphate.

Aldolase

A lyase enzyme that catalyzes step 4 of glycolysis, cleaving 6-carbon fructose 1,6-bisphosphate into two 3-carbon products: dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P).

Triose Phosphate Isomerase

An isomerase enzyme catalyzing step 5 of glycolysis, rapidly converting dihydroxyacetone phosphate (DHAP) into glyceraldehyde 3-phosphate (G3P) for the pay-off phase.

Glyceraldehyde 3-Phosphate Dehydrogenase

An oxidoreductase enzyme that catalyzes step 6 of glycolysis, carrying out a redox reaction coupled to inorganic phosphate addition to form 1,3-bisphosphoglycerate and $NADH$.

Phosphoglycerate Kinase

A transferase enzyme that catalyzes step 7 of glycolysis, carrying out substrate-level phosphorylation to transfer a high-energy phosphate from 1,3-bisphosphoglycerate to $ADP$, yielding $ATP$ and 3-phosphoglycerate.

Phosphoglycerate Mutase

An isomerase enzyme that catalyzes step 8 of glycolysis, shifting the phosphate group from carbon-3 to carbon-2 to convert 3-phosphoglycerate into 2-phosphoglycerate.

Enolase

A lyase enzyme catalyzing step 9 of glycolysis, which dehydrates 2-phosphoglycerate by removing $H_2O$ to generate the high-energy double bond of phosphoenolpyruvate (PEP).

Pyruvate Kinase

A transferase enzyme catalyzing step 10 of glycolysis, transferring a phosphate group from phosphoenolpyruvate to $ADP$ via substrate-level phosphorylation to generate $ATP$ and pyruvate.

Substrate-Level Phosphorylation

The direct synthesis of $ATP$ or $GTP$ through the enzymatic transfer of a phosphate group from a phosphorylated metabolic intermediate directly to $ADP$ or $GDP$.

Lactate Dehydrogenase

An enzyme that reduces pyruvate to lactate under anaerobic conditions, concurrently oxidizing $NADH$ back to $NAD^+$ to maintain cellular redox balance and sustain glycolysis.

Pyruvate Dehydrogenase Complex

A multi-enzyme complex ($E_1 + E_2 + E_3$) catalyzing the link reaction (step 0), which converts 3-carbon pyruvate into 2-carbon acetyl-CoA and $CO_2$ via oxidative decarboxylation while reducing $NAD^+$ to $NADH$.

Citrate Synthase

The transferase enzyme that catalyzes step 1 of the citric acid cycle, condensing 4-carbon oxaloacetate with the 2-carbon acetyl group of acetyl-CoA to form 6-carbon citrate.

Aconitase

An isomerase enzyme utilizing an $Fe^{2+}$ cofactor that catalyzes the reversible dehydration and hydration of citrate into isocitrate via the transient intermediate cis-aconitate.

Isocitrate Dehydrogenase

An oxidoreductase enzyme catalyzing step 3 of the citric acid cycle, which oxidizes isocitrate to oxalosuccinate and decarboxylates it to produce 5-carbon $\alpha$-ketoglutarate, $CO_2$, and $NADH$.

$\alpha$-Ketoglutarate Dehydrogenase Complex

An enzyme complex catalyzing step 4 of the citric acid cycle, performing the oxidative decarboxylation of $\alpha$-ketoglutarate ($5C$) with $CoA\text{-}SH$ to form succinyl-CoA ($4C$), $CO_2$, and $NADH$.

Succinyl-CoA Synthetase

A ligase enzyme that catalyzes step 5 of the citric acid cycle, breaking the high-energy thioester bond of succinyl-CoA to produce succinate and drive substrate-level phosphorylation of $GDP$ to $GTP$ (or $ATP$).

Succinate Dehydrogenase

An oxidoreductase enzyme located in the inner mitochondrial membrane (Complex II) that oxidizes succinate to fumarate while reducing $FAD$ to $FADH_2$.

Fumarase

A lyase enzyme catalyzing step 7 of the citric acid cycle, which stereospecifically hydrates the trans double bond of fumarate via a carbanion transition state to form L-malate.

Malate Dehydrogenase

An oxidoreductase enzyme catalyzing step 8 of the citric acid cycle, which oxidizes L-malate to regenerate oxaloacetate while reducing $NAD^+$ to $NADH + H^+$.

Mitochondrion

The organelle composed of an outer membrane, intermembrane space, folded cristae, and an inner matrix, where the citric acid cycle and oxidative phosphorylation occur.

Electron Transport Chain

A series of multi-protein complexes (I–IV) and mobile carriers embedded in the inner mitochondrial membrane that transfer electrons from $NADH$ and $FADH_2$ to reduce $O_2$ into $H_2O$.

Proton Motive Force (PMF)

An electrochemical potential gradient generated across the inner mitochondrial membrane as complexes I, III, and IV pump protons ($H^+$) from the matrix into the intermembrane space.

ATP Synthase

A rotary multi-subunit enzyme complex embedded in the inner mitochondrial membrane that harnesses proton flow down the electrochemical gradient to synthesize $ATP$ from $ADP$ and $P_i$.

Oxidative Phosphorylation

The final stage of aerobic respiration wherein the oxidation of $NADH$ and $FADH_2$ through the electron transport chain is chemiosmotically coupled to the phosphorylation of $ADP$ to form $ATP$.