Glycolysis and the Citric Acid Cycle

Vocabulary practice flashcards covering the stages of catabolism, the ten steps and enzymes of glycolysis, enzyme classifications, the pyruvate dehydrogenase complex, and the eight steps of the citric acid cycle (TCA cycle).

Stage 1 of Catabolism

The breakdown of large food molecules (proteins, polysaccharides, and fats) into their simple subunits (amino acids, simple sugars, fatty acids, and glycerol).

Stage 2 of Catabolism

The breakdown of simple subunits into acetyl $CoA$, during which limited amounts of $ATP$ and $NADH$ are produced in the cytosol and mitochondria.

Stage 3 of Catabolism

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

Glycolysis

A 10-step metabolic pathway in the cytosol (from Greek "glyk" meaning sweet and "lysis" meaning dissolution) that breaks down one molecule of glucose into two molecules of pyruvate, yielding a net of $2\text{ ATP}$ and $2\text{ NADH}$.

Hexokinase

The enzyme that catalyzes Step 1 of glycolysis, phosphorylating glucose using $ATP$ to produce glucose 6-phosphate, thereby trapping the sugar inside the cell via its negative charge.

Phosphoglucose Isomerase

The enzyme that catalyzes Step 2 of glycolysis, performing a reversible isomerization that rearranges glucose 6-phosphate (an aldose) into fructose 6-phosphate (a ketose).

Phosphofructokinase

The key regulatory enzyme of glycolysis that catalyzes Step 3, phosphorylating fructose 6-phosphate using $ATP$ to generate fructose 1,6-bisphosphate.

Aldolase

The enzyme that catalyzes Step 4 of glycolysis, cleaving the six-carbon sugar fructose 1,6-bisphosphate into two distinct three-carbon sugars: dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.

Triose Phosphate Isomerase

The enzyme that catalyzes Step 5 of glycolysis, reversibly isomerizing dihydroxyacetone phosphate into a second molecule of glyceraldehyde 3-phosphate.

Glyceraldehyde 3-Phosphate Dehydrogenase

The enzyme that catalyzes Step 6 of glycolysis, oxidizing glyceraldehyde 3-phosphate using $NAD^+$ and inorganic phosphate to produce 1,3-bisphosphoglycerate and $NADH$.

Phosphoglycerate Kinase

The enzyme that catalyzes Step 7 of glycolysis, transferring a high-energy phosphate group from 1,3-bisphosphoglycerate to $ADP$ to form $ATP$ and 3-phosphoglycerate.

Phosphoglycerate Mutase

The enzyme that catalyzes Step 8 of glycolysis, shifting the remaining phosphate ester linkage from carbon 3 to carbon 2 of 3-phosphoglycerate to form 2-phosphoglycerate.

Enolase

The enzyme that catalyzes Step 9 of glycolysis, removing a molecule of water from 2-phosphoglycerate to create a high-energy enol phosphate linkage in phosphoenolpyruvate.

Pyruvate Kinase

The enzyme that catalyzes Step 10 of glycolysis, transferring the high-energy phosphate group from phosphoenolpyruvate to $ADP$ to form $ATP$ and pyruvate.

Kinase

An enzyme type that catalyzes the addition of a phosphate group to molecules; transfers phosphate from $ATP$ to a substrate or transfers phosphate from a high-energy substrate to $ADP$ to form $ATP$.

Isomerase

An enzyme type that catalyzes the rearrangement of bonds within a single molecule to prepare it for subsequent chemical reactions.

Dehydrogenase

An enzyme type that catalyzes the oxidation of a molecule by removing a hydrogen atom plus an electron (a hydride ion, $H^-$), transferring them to electron carriers like $NAD^+$ or $FAD$.

Mutase

An enzyme type that catalyzes the shifting of a chemical group from one position to another within the same molecule.

Pyruvate Dehydrogenase Complex

A large multi-enzyme complex in the mitochondrial matrix containing pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase that converts pyruvate to acetyl $CoA$, $CO_2$, and $NADH$.

Citrate Synthase

The enzyme in Step 1 of the citric acid cycle that catalyzes the condensation of acetyl $CoA$ and oxaloacetate through an $S\text{-citryl-CoA}$ intermediate to form citrate, driven forward by the hydrolysis of the coenzyme A thioester bond.

Aconitase

The enzyme in Step 2 of the citric acid cycle that isomerizes citrate into isocitrate via a $cis\text{-aconitate}$ intermediate by consecutively removing and adding back water.

Isocitrate Dehydrogenase

The enzyme in Step 3 of the citric acid cycle that catalyzes the oxidation of isocitrate to $\alpha\text{-ketoglutarate}$ via an unstable oxalosuccinate intermediate, releasing $CO_2$ and producing $NADH$.

$\alpha$-Ketoglutarate Dehydrogenase Complex

The enzyme complex in Step 4 of the citric acid cycle that oxidatively decarboxylates $\alpha\text{-ketoglutarate}$ to form succinyl $CoA$, $CO_2$, and $NADH$.

Succinyl CoA Synthetase

The enzyme in Step 5 of the citric acid cycle that converts succinyl $CoA$ to succinate, using the energy of the thioester linkage to convert inorganic phosphate and $GDP$ to $GTP$ (or $ATP$ in bacteria and plants).

Succinate Dehydrogenase

The enzyme in Step 6 of the citric acid cycle that oxidizes succinate to fumarate by transferring two hydrogen atoms directly to $FAD$, producing $FADH_2$.

Fumarase

The enzyme in Step 7 of the citric acid cycle that catalyzes the hydration of fumarate to produce malate.

Malate Dehydrogenase

The enzyme in Step 8 of the citric acid cycle that catalyzes the final oxidation of malate to regenerate oxaloacetate, reducing $NAD^+$ to $NADH + H^+$.

Net Result of the Citric Acid Cycle

One complete turn of the citric acid cycle using one acetyl group yields $3\text{ NADH}$, $1\text{ GTP}$, and $1\text{ FADH}_2$, while releasing $2\text{ molecules of } CO_2$.

Mitochondrion

A double-membrane enclosed eukaryotic organelle comprising an outer membrane, an intermembrane space, a highly folded inner membrane, and an internal matrix where the citric acid cycle and pyruvate oxidation occur.

Oxidative Phosphorylation

The metabolic process on the inner mitochondrial membrane where high-energy electrons from $NADH$ and $FADH_2$ are passed down an electron transport chain to $O_2$, driving proton pumping to power $ATP$ synthesis.