Exam 3 Review

Covers oxidative phosphorylation, pathways related to glycolysis, and lipid metabolism.

Oxidative phosphorylation

The process occurring in the inner mitochondrial membrane by which cells produce ATP using energy released from electrons transferred through the electron transport chain (ETC).
-Takes place in the inner mitochondrial membrane in eukaryotic cells
-Uses electrons from NADH and FADH2
-Electrons move through a series of protein complexes.
-Energy released pumps protons (H+) across the inner mitochondrial membrane
-The resulting proton gradient powers ATP synthase to produce ATP
-Oxygen (O2) is the final electron acceptor
-Main purpose = convert energy stored in NADH and FADH2 into ATP.

Proton gradient

A difference in $H^+$ concentration across a membrane, generated by proton pumping across the inner mitochondrial membrane.

Chemiosmosis

The process through which protons ($H^+$) move down their electrochemical gradient through ATP synthase to drive ATP production.

How it works:
1.) Complexes I, III, and IV pump H+ from the mitochondrial matrix into the intermembrane space.
2.) Creates an electrochemical gradient.
3.) H+ flows back into the mitochondrial matrix through ATP synthase.
4.) The movement of protons causes ATP synthase to rotate and change shape.
5.) ATP synthase combines ADP + Pi to produce ATP.

ADP + Pi → ATP + H2O

ATP synthase

An enzyme consisting of a membrane-embedded $F_0$ portion and a catalytic $F_1$ portion that synthesizes ATP from $ADP + P_i$ using proton-motive force…. basically an enzyme that produces ATP using proton-motive force.

Complex I (NADH dehydrogenase)

An ETC complex that accepts electrons from NADH, oxidizes NADH to $NAD^+$, transfers electrons to ubiquinone (CoQ), and pumps $4\,H^+$ into the intermembrane space.

Complex II (Succinate dehydrogenase)

An enzyme complex functioning in both the citric acid cycle and ETC that accepts electrons from $FADH_2$, FADH2 is oxidized to FAD, transfers electrons to ubiquinone, and does not pump protons. Also functions in the citric acid cycle.

Complex III (Cytochrome $bc_1$ complex)

An ETC complex that receives electrons from reduced ubiquinone ($QH_2$), transfers electrons to cytochrome c, and pumps $4\,H^+$ into the intermembrane space.

Complex IV (Cytochrome c oxidase)

An ETC complex that receives electrons from cytochrome c, transfers them to oxygen to form water, and pumps $2\,H^+$ per pair of electrons.

Coenzyme Q (ubiquinone)

A mobile electron carrier that transfers electrons from Complex I and Complex II to Complex III.

Cytochrome c

A mobile electron carrier that shuttles electrons from Complex III to Complex IV.

Proton-motive force

Stored energy across the inner mitochondrial membrane created by a chemical gradient ($H^+$ concentration difference) and an electrical gradient (charge difference).

Oligomycin

An inhibitor that targets the $F_0$ portion of ATP synthase, preventing proton flow and stopping ATP synthesis.

Uncouplers

Allow protons to cross the inner mitochondrial membrane without passing through ATP synthase.

Effects:

• Proton gradient decreases.

• ATP production decreases.

• Energy is released as heat.

• Electron transport and oxygen consumption can increase.

Glycolysis

A 10-step cytosolic pathway that breaks down one glucose molecule into two molecules of pyruvate, producing a net yield of $2\text{ ATP}$ and $2\text{ NADH}$.

• Occurs in the cytoplasm (cytosol).

• Does not directly require oxygen.

• Consists of 10 enzyme-catalyzed reactions.

• Produces ATP and NADH.

• Can occur under both aerobic and anaerobic conditions.

Overall reaction:
Glucose + 2NAD+ + 2ADP + 2Pi → 2 Pyruvate + 2 NADH + 2H+ + 2ATP + 2H2O

Hexokinase

The glycolytic enzyme catalyzing the first irreversible step: the phosphorylation of glucose to glucose-6-phosphate.

Phosphofructokinase-1 (PFK-1)

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

Pyruvate kinase

The glycolytic enzyme catalyzing the irreversible tenth step: the transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP, producing pyruvate and ATP.

Pyruvate dehydrogenase complex (PDH)

A mitochondrial multi-enzyme complex that converts pyruvate to acetyl-CoA, $CO_2$, and NADH; utilizes TPP, lipoamide, Coenzyme A, FAD, and NAD as cofactors.

Citric Acid Cycle (Krebs Cycle / TCA Cycle)

A series of reactions in the mitochondrial matrix that oxidizes acetyl-CoA into two molecules of $CO_2$, yielding $3\text{ NADH}$, $1\text{ FADH}_2$, and $1\text{ GTP}$ (or ATP) per acetyl-CoA.

Fermentation

An anaerobic metabolic pathway that regenerates $NAD^+$ from NADH so that glycolysis can continue, without directly producing additional ATP.

Gluconeogenesis

The synthesis of glucose from non-carbohydrate precursors (such as lactate, glycerol, and glucogenic amino acids) occurring mainly in the liver and kidneys.

Pyruvate carboxylase

A biotin- and ATP-dependent gluconeogenic enzyme in mitochondria that converts pyruvate into oxaloacetate; activated by acetyl-CoA.

Fructose-1,6-bisphosphatase

A gluconeogenic bypass enzyme that hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate; inhibited by AMP and fructose-2,6-bisphosphate.

Glucose-6-phosphatase

A gluconeogenic enzyme in the liver and kidneys (absent in skeletal muscle) that converts glucose-6-phosphate into free glucose.

Pentose Phosphate Pathway (PPP)

A cytosolic alternative pathway for glucose-6-phosphate metabolism that produces NADPH for biosynthesis and antioxidant defense, as well as ribose-5-phosphate for nucleotide synthesis.

Glucose-6-phosphate dehydrogenase (G6PD)

The first and rate-limiting enzyme of the oxidative phase of the pentose phosphate pathway; produces NADPH and is inhibited by NADPH.

Lipolysis

The enzymatic hydrolysis of triglycerides into glycerol and three free fatty acids, carried out by ATGL, hormone-sensitive lipase (HSL), and monoacylglycerol lipase.

Carnitine shuttle

A mitochondrial transport system involving CPT-I, translocase, and CPT-II that moves long-chain fatty acyl groups across the inner mitochondrial membrane into the matrix.

Carnitine palmitoyltransferase I (CPT-I)

An outer mitochondrial membrane enzyme that transfers fatty acyl groups from CoA to carnitine; allosterically inhibited by malonyl-CoA.

\beta-Oxidation

A repeating four-step mitochondrial pathway (oxidation, hydration, oxidation, thiolysis) that breaks down fatty acyl-CoA into acetyl-CoA, NADH, and $FADH_2$.

Acetyl-CoA carboxylase (ACC)

The rate-limiting, biotin-dependent cytosolic enzyme of fatty acid synthesis that converts acetyl-CoA and $CO_2$ into malonyl-CoA using ATP.

Fatty acid synthase (FAS)

A cytosolic enzyme complex that uses an acetyl-CoA primer, malonyl-CoA donors, and NADPH reducing equivalents to synthesize palmitate ($16:0$).

Ketone bodies

Water-soluble fuel molecules (acetoacetate, $\beta$-hydroxybutyrate, and acetone) synthesized in liver mitochondria from acetyl-CoA during fasting or low carbohydrate availability.

HMG-CoA reductase

The rate-limiting enzyme of cholesterol synthesis in the cytosol and smooth ER that converts HMG-CoA into mevalonate.

Statins

Pharmaceutical agents that competitively inhibit HMG-CoA reductase, thereby lowering endogenous cholesterol synthesis and upregulating hepatic LDL receptors.

Chylomicrons

Lipoprotein particles containing ApoB-48 that transport dietary triglycerides and cholesterol from the intestine to peripheral tissues.

Lipoprotein lipase (LPL)

A capillary endothelial enzyme activated by ApoC-II that hydrolyzes triglycerides within circulating chylomicrons and VLDL into free fatty acids and glycerol.

Glycogenesis

The biochemical pathway converting glucose into glycogen for energy storage in the liver and skeletal muscle, catalyzed primarily by glycogen synthase.

Glycogenolysis

The breakdown of glycogen into glucose-1-phosphate, primarily catalyzed by glycogen phosphorylase and stimulated by glucagon in the liver and epinephrine in muscle.

Cori cycle

A metabolic pathway in which lactate produced by muscle glycolysis travels through blood to the liver to be converted into glucose via gluconeogenesis, which then returns to muscle.

Terminal electron acceptor

Oxygen, which accepts electrons at the end of the ETC.

Proton-motive Force

Is the stored energy created by:
-A difference in H+ concentration (chemical gradient)

-A difference in electrical charge across the membrane (electrical gradient)

Both contribute to ATP production.

Why is NADH worth more ATP?

• NADH donates electrons to Complex I.

• FADH₂ donates electrons to Complex II.

• Complex II does not pump protons.

• Therefore, FADH₂ results in fewer protons pumped and less ATP produced.

Inhibitors and their targets?

Rotenone → Complex I
Antimycin A → Complex III
Cyanide → Complex IV
Carbon Monoxide → IV
Oligomycin → ATP synthase

What do ETC inhibitors do?

prevent electron transfer through specific complexes. Is to important b/c they affect ATP production.

What happens when the ETC is inhibited?

• Electron transport slows or stops.

• Proton pumping decreases or stops.

• The proton gradient eventually dissipates.

• ATP production through oxidative phosphorylation decreases.

• NADH oxidation is impaired.

Inhibitors vs Uncouplers?

• Inhibitors stop electron transport or ATP synthesis.

• Uncouplers dissipate the proton gradient, separating electron transport from ATP synthesis.