Cellular Respiration: ATP Synthesis Pathways

Citric Acid Cycle

The citric acid cycle is a series of eight reactions in which an acetyl group is decarboxylated and oxidized, generating:

• 3 molecules of NADH
• 1 molecule of FADH2
• 2 molecules of CO2
• 1 molecule of ATP (or GTP)

For each molecule of glucose entering glycolysis, two molecules of pyruvate are produced. Pyruvate is then converted to two molecules of acetyl CoA, which can enter this cycle.

Oxygen is used indirectly in cellular respiration. Glycolysis breaks down a glucose molecule, generating two molecules of pyruvate. Pyruvate moves into the mitochondrial matrix. Inside the matrix, pyruvate is converted to acetyl CoA, which then enters the citric acid cycle.

Key steps in the cycle include:

• Acetyl CoA reacts with a molecule of oxaloacetate to form citrate via a condensation reaction.
• Citrate is converted back to oxaloacetate through a series of reactions. During this process, two carbon atoms from the acetyl group are released as CO2.

The cycle consumes one acetyl CoA molecule and produces two CO2 molecules. It also consumes three NAD+ and one FAD, producing three NADH + 3 H+ and one FADH2.

Each glucose molecule yields two acetyl CoA molecules, resulting in two turns of the cycle. The net yield per glucose from the citric acid cycle is 4 CO2 + 2 ATP (or GTP) + 6 NADH + 2 FADH2.

The citric acid cycle also provides intermediates for biosynthesis. Many amino acids are synthesized from alpha-ketoglutarate and oxaloacetate. Oxaloacetate can also be converted to glucose depending on the needs of the organism.

The electron transport chain and oxidative phosphorylation use electrons stored in NADH and FADH2 to drive the synthesis of ATP. The energy released in electron transport generates an electrochemical gradient that is used in oxidative phosphorylation of ADP to produce ATP.

Electron Transport Chain

The electron transport chain consists of a series of protein complexes embedded in the inner mitochondrial membrane.

Complex I

• Contains approximately 40 subunits, including a flavoprotein, several iron-sulfur (Fe-S) clusters, and coenzyme Q (CoQ, ubiquinone) binding sites.
• Complex I oxidizes NADH to NAD+.
• CoQ is the oxidizing agent, which is reduced to CoQH2.
• Some of the energy from this reaction is used to pump 4 H+ from the matrix to the intermembrane space.

Reaction: NADH + H+ + CoQ → NAD+ + CoQH2 + 4 H+ (pumped)

Complex II

• Oxidizes FADH2 to FAD.
• CoQ is the oxidizing agent, which is reduced to CoQH2.
• This complex does not pump protons.

Reaction: FADH2 + CoQ → FAD + CoQH2

Complex III

• An integral membrane complex containing 11 subunits, including cytochrome b, cytochrome c1, and Fe-S clusters.
• Complex III delivers electrons from CoQH2 to cytochrome c (Cyt c).
• Complex III pumps 4 H+ from the matrix to the intermembrane space.

Reaction: CoQH2 + 2 Cyt c (oxidized) → CoQ + 2 Cyt c (reduced) + 4 H+ (pumped)

Complex IV (Cytochrome Oxidase)

• Contains 13 subunits, including cytochrome a and a3.
• Electrons flow from Cyt c in complex III to Cyt a3 in complex IV.
• Electrons are finally transferred from Cyt a3 to molecular oxygen (O2), which is the final electron acceptor, forming water.
• Some of the energy from this reaction is used to pump 2 H+ from the matrix to the intermembrane space.

Reaction: O2 + 4 H+ + 4 e- → 2 H2O + 2 H+ (pumped)

Adding the proton pumping from complexes I, III, and IV: 10 H+ are pumped per NADH molecule oxidized, and 6 H+ are pumped per FADH2 molecule oxidized (Complex II feeds electrons to CoQ, bypassing Complex I).

Oxidative Phosphorylation

Oxidative phosphorylation is the process where ATP is synthesized using the energy released by the movement of protons across the inner mitochondrial membrane. Protons flow back into the mitochondrial matrix down their electrochemical gradient through ATP synthase. This flow of protons causes the rotation of the ATP synthase structure. This mechanical energy is converted into chemical energy necessary for the formation of ATP from ADP and Pi.

Coupling Oxidation & Phosphorylation

The net reactions of oxidative phosphorylation, linking electron transport (oxidation) to ATP synthesis (phosphorylation), are approximately:

• NADH + H+ + 1/2 O2 + ~2.5 ADP + ~2.5 Pi → NAD+ + H2O + ~2.5 ATP
• FADH2 + 1/2 O2 + ~1.5 ADP + ~1.5 Pi → FAD + H2O + ~1.5 ATP

Using older estimates (often used for simplified calculations):

• 3 NADH x 3 ATP/NADH = 9 ATP
• 1 FADH2 x 2 ATP/FADH2 = 2 ATP
• 1 GTP = 1 ATP

From one turn of the citric acid cycle (per acetyl CoA): 3 NADH + 1 FADH2 + 1 GTP → 9 + 2 + 1 = 12 ATP.

Complete Glucose Oxidation Energy

The total ATP yield from the complete oxidation of one glucose molecule through cellular respiration (using older ATP yield estimates):

Glycolysis

Glucose → 2 Pyruvate + 2 ATP (net) + 2 NADH

ATP yield: 2 ATP + (2 NADH x 3 ATP/NADH) = 2 + 6 = 8 ATP

Pyruvate Oxidation

2 Pyruvate → 2 Acetyl CoA + 2 CO2 + 2 NADH

ATP yield: (2 NADH x 3 ATP/NADH) = 6 ATP

Citric Acid Cycle (2 turns per glucose)

2 Acetyl CoA → 4 CO2 + 6 NADH + 2 FADH2 + 2 GTP

ATP yield: (6 NADH x 3 ATP/NADH) + (2 FADH2 x 2 ATP/FADH2) + (2 GTP x 1 ATP/GTP) = 18 + 4 + 2 = 24 ATP

Total ATP per glucose: 8 (Glycolysis) + 6 (Pyruvate Oxidation) + 24 (Citric Acid Cycle) = 38 ATP.

Note: Actual ATP yields can vary due to factors like proton leak and the cost of transporting molecules into the mitochondria. Modern estimates often use ~2.5 ATP per NADH and ~1.5 ATP per FADH2, resulting in a lower total yield (e.g., ~30-32 ATP).

Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. It occurs mainly in the liver and, to a lesser extent, the kidneys. It is a series of reactions that convert molecules like pyruvate, lactate, glycerol, and certain amino acids into glucose.

Cori Cycle

The Cori cycle describes the metabolic pathway in which lactate produced by anaerobic glycolysis in muscles is transported to the liver and converted to glucose. The glucose is then returned to the muscles to be used for energy or stored as glycogen. This cycle allows muscles to continue anaerobic activity and shifts the metabolic burden of lactate disposal to the liver.

Glucose-Alanine Cycle

The glucose-alanine cycle is another inter-organ metabolic pathway, primarily between muscle and liver. In muscle, pyruvate (from glycolysis) can be transaminated to alanine, accepting an amino group from amino acid breakdown. Alanine is then transported to the liver, where it is converted back to pyruvate, which can be used for gluconeogenesis. The amino group is processed into urea in the liver. This cycle complements the Cori cycle by transporting nitrogen from muscle to liver while also providing a substrate for glucose synthesis.

Pentose Phosphate Pathway

The pentose phosphate pathway (also known as the phosphogluconate pathway or hexose monophosphate shunt) is a metabolic pathway parallel to glycolysis. Its main functions are:

• Generating NADPH, which is essential for reductive biosynthesis (e.g., fatty acid synthesis, cholesterol synthesis) and protecting against oxidative stress.
• Generating ribose-5-phosphate, a precursor for the synthesis of nucleotides (DNA and RNA) and coenzymes (ATP, NADH, FADH2, CoA).