Glycolysis and the Citric Acid (TCA) Cycle

Comprehensive vocabulary flashcards covering the stages of cellular respiration, all ten steps and enzymes of glycolysis, enzyme classifications, the pyruvate dehydrogenase complex, and the eight steps of the citric acid (TCA) cycle based on University of Leeds lecture notes.

Stage 1 of Food Breakdown

The initial stage of catabolism where large food molecules are digested into simple subunits: proteins into amino acids, polysaccharides into simple sugars, and fats into fatty acids and glycerol.

Stage 2 of Food Breakdown

The catabolic stage involving the breakdown of simple subunits to acetyl CoA, which includes glycolysis in the cytosol and yields limited amounts of $ATP$ and $NADH$.

Stage 3 of Food Breakdown

The complete oxidation of the acetyl group in acetyl CoA to $H_2O$ and $CO_2$, generating large amounts of $ATP$ on the inner mitochondrial membrane via oxidative phosphorylation.

Glycolysis

Derived from the Greek "glyk" (sweet) and "lysis" (dissolution), this is the cytosolic pathway converting one molecule of glucose into two molecules of pyruvate with a net result of $2$ pyruvate, $2$ $ATP$, and $2$ $NADH$.

Hexokinase

The enzyme in step $1$ of glycolysis that phosphorylates glucose using $ATP$ to form glucose 6-phosphate, trapping the negatively charged sugar phosphate inside the cell.

Phosphoglucose Isomerase

The enzyme in step $2$ of glycolysis that catalyzes the reversible isomerization of glucose 6-phosphate (an aldose) to fructose 6-phosphate (a ketose) by moving the carbonyl oxygen from carbon $1$ to carbon $2$.

Phosphofructokinase

The key regulatory enzyme in step $3$ of glycolysis that controls sugar entry into the pathway by phosphorylating the hydroxyl group on carbon $1$ of fructose 6-phosphate using $ATP$ to produce fructose 1,6-bisphosphate.

Aldolase

The enzyme in step $4$ of glycolysis that cleaves the six-carbon sugar fructose 1,6-bisphosphate into two three-carbon sugars: dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.

Triose Phosphate Isomerase

The enzyme in step $5$ of glycolysis that rapidly isomerizes dihydroxyacetone phosphate into a second molecule of glyceraldehyde 3-phosphate.

Glyceraldehyde 3-Phosphate Dehydrogenase

The enzyme in step $6$ of glycolysis that oxidizes glyceraldehyde 3-phosphate using inorganic phosphate and $NAD^+$ to form 1,3-bisphosphoglycerate and $NADH + H^+$, marking the start of the energy-generation phase.

Phosphoglycerate Kinase

The enzyme in step $7$ of glycolysis that transfers the high-energy phosphate group from 1,3-bisphosphoglycerate to $ADP$, generating $ATP$ and 3-phosphoglycerate.

Phosphoglycerate Mutase

The enzyme in step $8$ of glycolysis that moves the phosphate ester linkage from carbon $3$ to carbon $2$, converting 3-phosphoglycerate to 2-phosphoglycerate.

Enolase

The enzyme in step $9$ of glycolysis that removes a molecule of water from 2-phosphoglycerate to produce a high-energy enol phosphate linkage in phosphoenolpyruvate.

Pyruvate Kinase

The enzyme in step $10$ of glycolysis that transfers the high-energy phosphate group from phosphoenolpyruvate to $ADP$, producing $ATP$ and pyruvate.

Kinase

An enzyme class that catalyzes the addition of a phosphate group to molecules, such as transferring phosphate from $ATP$ to a substrate (steps $1$ and $3$) or to $ADP$ to form $ATP$ (steps $7$ and $10$).

Isomerase

An enzyme class that catalyzes the rearrangement of bonds within a single molecule to prepare it for subsequent chemical alterations (steps $2$ and $5$ of glycolysis).

Dehydrogenase

An enzyme class that catalyzes the oxidation of a molecule by removing a hydrogen atom plus an electron (a hydride ion, $H^-$), such as generating $NADH$ in step $6$ of glycolysis.

Mutase

An enzyme class that catalyzes the shifting of a chemical group from one position to another within a single molecule (e.g., shifting phosphate in step $8$ of glycolysis).

Pyruvate Dehydrogenase Complex

A large multi-enzyme complex located in the mitochondrial matrix that converts pyruvate into acetyl CoA and $CO_2$ while reducing $NAD^+$ to $NADH + H^+$.

Three Enzymes of the Pyruvate Dehydrogenase Complex

The three enzymatic components present in multiple copies within the complex: ($1$) pyruvate dehydrogenase, ($2$) dihydrolipoyl transacetylase, and ($3$) dihydrolipoyl dehydrogenase.

Citric Acid Cycle (TCA Cycle)

A cyclic eight-step pathway in the mitochondrial matrix that completely oxidizes the two-carbon acetyl group of acetyl CoA to $CO_2$, producing three $NADH$, one $GTP$, and one $FADH_2$, and releasing two molecules of $CO_2$ per turn.

Citrate Synthase

The enzyme in step $1$ of the citric acid cycle that combines the methyl carbon of acetyl CoA with the carbonyl carbon of oxaloacetate to produce citrate via an S-citryl-CoA intermediate, driven by the hydrolysis of $HS\text{-}CoA$.

Aconitase

The enzyme in step $2$ of the citric acid cycle that catalyzes the isomerization of citrate to isocitrate via a cis-aconitate intermediate by sequentially removing and adding back water.

Isocitrate Dehydrogenase

The enzyme in step $3$ of the citric acid cycle that catalyzes the first oxidation step, converting isocitrate to $\alpha$-ketoglutarate via an unstable oxalosuccinate intermediate, releasing $CO_2$ and producing $NADH + H^+$.

\alpha-Ketoglutarate Dehydrogenase Complex

The enzyme complex in step $4$ of the citric acid cycle that catalyzes the oxidative decarboxylation of $\alpha$-ketoglutarate to succinyl CoA, generating $NADH$, $CO_2$, and a high-energy thioester bond to coenzyme A.

Succinyl CoA Synthetase

The enzyme in step $5$ of the citric acid cycle that displaces CoA with an inorganic phosphate to form a high-energy phosphate linkage, which is transferred to $GDP$ to produce $GTP$ and succinate.

Succinate Dehydrogenase

The enzyme in step $6$ of the citric acid cycle that catalyzes the oxidation of succinate to fumarate by transferring two hydrogen atoms to $FAD$, forming $FADH_2$.

Fumarase

The enzyme in step $7$ of the citric acid cycle that catalyzes the hydration of fumarate by adding water across its double bond to form malate.

Malate Dehydrogenase

The enzyme in step $8$ of the citric acid cycle that catalyzes the final oxidation step, converting malate to oxaloacetate and yielding $NADH + H^+$.

Oxidative Phosphorylation

The final stage of energy extraction on the inner mitochondrial membrane where electrons from $NADH$ and $FADH_2$ pass through an electron-transport chain to $O_2$, pumping protons ($H^+$) to drive the mass synthesis of $ATP$.