Bio 211 Intro Molecular and Cell Biology
Lecture 15 “Electron Transport/Oxidative Phosphorylation”
Reading: Campbell Chap. 9 pp. 156-166
So far we have looked at two of the processes that contribute to cell respiration and enable the cell to generate ATP: glycolysis and the Krebs cycle. Today, we will look at electron transport and oxidative phosphorylation, two processes critical for making ATP in high yield. In electron transport reduced substrates such as NADH pass their electrons through a series of carriers generating a proton gradient. In oxidative phosphorylation, this proton gradient is used to drive ATP synthesis.
Outline:
1. Redox reactions
2. Electron transport
3. Chemiosmosis
4. Energy yields from respiration
1. Redox reactions
In many chemical reactions, there is a transfer of one or more electrons from one reactant to another. These are called oxidation-reduction or redox reactions. In an oxidation-reduction reaction, one substance loses electrons; we say it is oxidized. The other substance gains electrons; we say it is reduced.
In the reaction:
Xe- + Y ----> X + Ye-
X is the electron donor or the reducing agent; Y is the electron acceptor or the oxidizing agent.
The overall process of respiration involves oxidation and reduction:
C6H12O6 + 6O2----> 6 CO2 + 6H2O
Glucose is oxidized and oxygen is reduced in the process.
Many of the individual reactions that comprise the process of respiration are oxidation-reduction reactions. In this section, we will be introduced to some of the molecules that are typically involved as electron carriers in redox reactions.
The name of one essential carrier is NAD+: nicotinamide adenine dinucleotide. The molecule consists of two nucleotides joined together. Fig. 9.4.
Oxidized NAD+ can carry two electrons and a proton donated from an organic substrate. We describe the reaction as:
NAD+ + 2e- + 2H+ ----> NADH + H+
oxidized reduced
The electrons and protons in the reaction are derived from food molecules. The enzymes that catalyze redox reactions involving NAD carriers are called dehydrogenases.
Chemical energy from reduced NADH is usually released by the processes of electron transport and oxidative phosphorylation, which we will describe in more detailin the next lecture. Essentially what happens is the electrons and protons are passed from reduced NADH through a series of electron carriers and ultimately to oxygen, forming water. The energy is conserved in these transfers and is used to make ATP.
2. Electron transport
In describing the processes of glycolysis and the Krebs cycle, we have discussed several stages where chemical energy is conserved as ATP. However, most ATP that is formed comes from the reduced electron carriers, NADH and FADH2 . How is the energy from these electron carriers conserved as chemical energy in the form of ATP? This happens through electron transport and oxidative phosphorylation, which we will describe next.
Electron transport is a linear pathway of electron carriers that takes place in the mitochondrial inner membrane. Figure 9.13.
NADH->FMN->FeS->Q->cyt.b->FeS->cyt.c1->cyt. c->cyt.a->cyt.a3->O2
NADH passes its electrons to FMN in an oxidation/reduction reaction.
NADH is oxidized in the reaction, while FMN is reduced.
Other carriers in the pathway include FeS proteins (iron-sulfur proteins), ubiquinone (Q) which is lipid soluble and a series of cytochromes which have heme as their prosthetic group and thus are like myoglobin or hemoglobin.
The ultimate electron acceptor is molecular oxygen which is converted to water upon addition of electrons and protons:
1/2 O2 + 2 H+ + 2 e- ----> H2O
How does this process of electron transport generate ATP?
3. Oxidative phosphorylation: The chemiosmotic hypothesis
The process of electron transport does not make ATP directly. ATP is made indirectly via a proton gradient. How the proton gradient is used to make ATP is called the chemiosmotic hypothesis and was originally proposed by Peter Mitchell in the early 1960’s.
As electrons are pumped during electron transport, protons are also pumped across the membrane. This makes the outer face of the membrane somewhat more acidic and the inner face somewhat more basic. The protons can only return through the membrane through channels of the ATP synthase, the enzyme that makes ATP from ADP. The proton movement creates a conformational change, rather like wind blowing a fan, and causes ATP to be synthesized by the enzyme.
Key features of the chemisosmotic hypothesis:
(1) requires an intact membrane
(2) proton pumping during electron transport creates a pH difference (called a proton gradient or a proton-motive force)
(3) protons move back across the membrane through the F0 portion of the ATP synthase
(4) proton movement drives ATP synthesis by the F1 portion of the ATP synthase
What happens if
(1) you have a leaky membrane?
(2) you block proton movement?
Either one inhibits ATP synthesis.
4. Energy summary
Figure 9.15 summarizes the processes of electron transport and oxidative phosphorylation as they occur in mitochondria. Carriers such as NADH pass their electrons to proteins in the membrane. Electrons pass from the carriers ultimately to reduce oxygen, forming water. Protons are also moved across the membrane by several of the carriers. This sets up a proton gradient. Protons move back across the membrane through the ATP synthase, where they drive formation of ATP from ADP and Pi.
How much energy is produced from the electron transport chain and oxidative phosphylation?
Each NADH yields about 3 ATP.
Each FADH2 yields about 2 ATP.
Figure 9.16 shows all the processes of glucose breakdown that lead to formation of ATP and the production of reduced carriers.
Glycolysis yields 2 ATP directly plus 2 NADH.
some ATP is used to move the NADH into the mitochondrion
Krebs cycle yields 2 ATP directly plus 6 NADH and 2 FADH2.
Total yield is about 36 ATP/per glucose oxidized by respiration. Of this, nearly all of it comes from electron transport and oxidative phosphorylation.
How efficient is respiration in deriving energy from glucose? We can estimate this based on the free energy changes of glucose oxidation and ATP synthesis.
C6H12O6 + 6 O2 ----> 6 H2O + 6 CO2G = -686 kcal/mol
36 ADP + 36 Pi ----> 36 ATP G = +262.8 kcal/mol
C6H12O6 + 6 O2 + 36 ADP + 36 Pi ---> 6 H2O + 6 CO2 +36 ATP G = -423.2 kcal/mol
The efficiency can be calculated from 262.8/686 x 100% = 38% efficiency, more efficient than the most fuel efficient car which uses about 25% of the energy from gasoline to generate movement of the car.
What happens to the rest of the energy bound up as glucose? Lost as heat.
Summary: In today’s class, we looked at how the processes of electron transport and oxidative phosphorylation contribute to ATP production in the cell. Most of the energy produced by oxidizing glucose in respiration is conserved by these two processes. In electron transport, electrons are passed through a series of carriers and ultimately to oxygen and protons are pumped across the membrane. In oxidative phosphorylation, the energy from electron transport is conserved as a proton gradient that drives the synthesis of ATP.
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