Glycolysis, The Krebs Cycle, and the ETC

Electron Transport and Chemiosmosis

Most of the ETC molecules are proteins containing chemical groups that facilitate redox reactions. All but one of these proteins are embedded in the inner mitochondrial membrane. In contrast, the lipid-soluble ubiquinone (Q) can move throughout the membrane. During electron transport, NADH donates electrons to a flavin-containing protein at the top of the chain, but FADH2 donates electrons to an iron-sulfur protein that passes electrons directly to Q.

ATP Yield from Cellular Respiration

The vast majority of the "payoff" from glucose oxidation occurs via oxidative phosphorylation; ATP synthase produces 25 of the 29 ATP molecules produced per glucose molecule during cell respiration.

Introducing ATP

ATP (adenosine triphosphate) is the cellular currency for energy - it provides the fuel for most cellular activities. ATP has high potential energy and allows cells to do work. ATP works by phosphorylating (transferring a phosphate group) target molecules.

Cellular respiration has four components:

1) Glycolysis 2) Pyruvate processing 3) The citric acid cycle 4) Electron transport and chemiosmosis

The Substrates of the Citric Acid Cycle

A series of carboxylic acids is oxidized and recycled in the citric acid cycle. Citrate (the first molecule in the cycle) is formed from Acetyl CoA and oxaloacetate (the last molecule in the cycle). The citric acid cycle completes glucose oxidation. The energy released by the oxidation of one acetyl CoA molecule is used to produce 3 NADH, 1 FADH2, and 1 GTP, which is then converted to ATP.

Cellular respiration produces _____ from molecules with high potential energy - often glucose.

ATP

ATP Synthase Structure

ATP synthase is an enzyme complex consisting of two components: An ATPase "knob" (F1 unit) A membrane-bound, proton-transporting base (F0 unit) The units are connected by a rotor, which spins the F1 unit, and a stator, which interacts with the spinning F1 unit. Protons flowing through the F0 unit spin the rotor. As the F1 unit spins, its subunits change shape, and catalyze the phosphorylation of ADP to ATP.

Feedback Inhibition Regulates Glycolysis

During glycolysis, high levels of ATP inhibit the enzyme phosphofructokinase, which catalyzes one of the early reactions. Phosphofructokinase has two binding sites for ATP: 1) The active site, where ATP phosphorylates fructose-6-phosphate, resulting in the synthesis of fructose-1,6-bisphosphate 2) A regulatory site High ATP concentrations cause ATP to bind at the regulatory site, changing the enzyme's shape and dramatically decreasing the reaction rate at the active site.

The Electron Transport Chain

During the fourth step in cellular respiration, the high potential energy of the electrons carried by NADH and FADH2 is gradually decreased as they move through a series of redox reactions. The proteins involved in these reactions make up what is called an electron transport chain (ETC). O2 is the final electron acceptor. The transfer of electrons along with protons to oxygen forms water.

The Citric Acid Cycle

During the third step of glucose oxidation, the acetyl CoA produced by pyruvate processing enters the citric acid cycle, located in the mitochondrial matrix. Each acetyl CoA is oxidized to two molecules of CO2. Some of the potential energy released is used to 1) Reduce NAD+ to NADH. 2) Reduce flavin adenine dinucleotide (FAD) to FADH2 (another electron carrier). 3) Phosphorylate GDP to form GTP (later converted to ATP).

How Is the Electron Transport Chain Organized?

ETC proteins are organized into four large multiprotein complexes (called complex I-IV) and cofactors. Protons are pumped into the intermembrane space from the mitochondrial matrix by complexes I and IV. Q and the protein cytochrome c transfer electrons between complexes. Q also carries protons across the membrane.

Free Energy Changes, NADH, and FADH2

For each glucose molecule that is oxidized to 6 CO2, the cell reduces 10 molecules of NAD+ to NADH and 2 molecules of FAD to FADH2, and produces 4 molecules of ATP by substrate-level phosphorylation. The ATP can be used directly for cellular work. However, most of glucose's original energy is contained in the electrons transferred to NADH and FADH2, which then carry them to oxygen, the final electron acceptor.

The Glycolysis Reactions

Glycolysis consists of an energy investment phase and an energy payoff phase. In the energy investment phase, two molecules of ATP are consumed, and glucose is phosphorylated twice, forming fructose-1,6-bisphosphate. In the energy payoff phase: Sugar is split to form two pyruvate molecules. Two molecules of NAD+ are reduced to NADH. Four molecules of ATP are formed by substrate-level phosphorylation (net gain of 2 ATP).

Glycolysis: Oxidation of Glucose to Pyruvate

Glycolysis, a series of 10 chemical reactions, is the first step in glucose oxidation. Occurs in the cytoplasm. All of the enzymes needed for glycolysis are found in the cytosol. In glycolysis, glucose is broken down into two 3-carbon molecules of pyruvate, and the potential energy released is used to phosphorylate ADP to form ATP.

Oxidative phosphorylation.

In an electron transport chain a proton gradient provides energy for ATP production; the membrane protein ATP synthase uses this energy to phosphorylate ADP to form ATP

Feedback Inhibition

Occurs when an enzyme in a pathway is inhibited by the product of that pathway. Cells that are able to stop glycolytic reactions when ATP is abundant can conserve their stores of glucose for times when ATP is scarce.

Pyruvate Processing

Pyruvate processing is the second step in glucose oxidation. It is catalyzed by the enzyme pyruvate dehydrogenase in the mitochondrial matrix. In the presence of O2, pyruvate undergoes a series of reactions that results in the product molecule acetyl coenzyme A (acetyl CoA). During these reactions, another molecule of NADH is synthesized, and one of the carbon atoms in pyruvate is oxidized to CO2.

Pyruvate Processing Regulation

Pyruvate processing is under both positive and negative control. Feedback Inhibition (ATP, NADH, and Acetyl CoA) Phosphorylation inactivates the enzyme High Pyruvate levels promote the dephosphorylation (activation) of the enzyme

The Remaining Reactions Occur in the Mitochondria

Pyruvate produced during glycolysis is transported from the cytosol into the mitochondria. Mitochondria have both inner and outer membranes. Layers of sac-like structures called cristae fill the interior of the mitochondria, and are connected to the inner membrane by short tubes. The mitochondrial matrix is inside the inner membrane but outside the cristae.

The Chemiosmotic Hypothesis

The ETC pumps protons from the mitochondrial matrix to the intermembrane space. The proton-motive force from this electrochemical gradient can be used to make ATP in a process known as chemiosmosis.

The Citric Acid Cycle Regulation and Summary

The citric acid cycle can be turned off at multiple points, via several different mechanisms of feedback inhibition. To summarize, the citric acid cycle starts with acetyl CoA and ends with CO2. The potential energy that is released is used to produce NADH, FADH2, and ATP. When energy supplies are high, the cycle slows down.

Oxidative Phosphorylation

The energy released as electrons move through the ETC is used to pump protons across the plasma membrane into the intermembrane space, forming a strong electrochemical gradient. The protons then move through the enzyme ATP synthase, driving the production of ATP from ADP and Pi. Because this mode of ATP production links the phosphorylation of ADP with NADH and FADH2 oxidation, it is called oxidative phosphorylation.

After glycolysis and the citric acid cycle are complete, where is most of the energy that was once contained in the bonds of glucose? // a. converted by the cell into ATP via substrate-level phosphorylation b. in NADH and FADH2 that are carrying energy-rich electrons c. in the CO2 that has been released d. converted by the cell into ATP via oxidative phosphorylation

b. in NADH and FADH2 that are carrying energy-rich electrons

Substrate Level Phosphorylation

occurs when ATP is produced by the enzyme-catalyzed transfer of a phosphate group from an intermediate substrate to ADP. (This is how ATP is produced in glycolysis and the citric acid cycle).