Module 7 - Oxidative Phosphorylation Flashcards
Binding-Change Mechanism
The cycling of the three β-subunits in ATP synthase between three conformational states; Open, Tight, and Loose. The active site in each state performs a different function.
Cellular Respiration
The set of metabolic reactions and processes that convert biochemical energy in fuels into ATP. This includes the citric acid cycle, electron transport, and oxidative phosphorylation.
Chemiosmotic Hypothesis
The proton gradient generated by the energy released during electron transport results in a proton-motive force, which allows ATP synthase to generate ATP from ADP and Pi.
Cytochromes
Proteins that contain a heme moiety and function in electron transfer.
Electrochemical Gradient
A gradient that consists of two parts, a chemical gradient which reflects the different in solute concentration across a membrane, and an electrical gradient which is the difference in charge across a membrane.
Proton-Motive Force
The force that promotes movement of protons across membranes from the side of highest concentration to the side with the lower concentration.
Reactive Oxygen Species
Chemically reactive and unstable atoms or molecules that contain oxygen, and which readily react with other molecules.
Redox Potential
The measure of the tendency of a molecule to acquire electrons and become reduced.
Where does oxidative phosphorylation take place?
In the mitochondria
Where does the citric acid cycle, which generates most of the NADH and FADH2 from the oxidation of fuels, occur?
in the matrix of the mitochondria, or the “inside” of the mitochondria
Redox potential E0’, also called the reduction potential
it is a measure of a molecule’s tendency to donate or accept electrons.
A strong reducing agent, such as NADH, readily __________ electrons and has a negative E0’.
donates
A strong oxidizing agent, such as O2, readily _________ electrons and has a positive E0’.
accepts
the electrons from NADH or FADH2 do not flow directly to oxygen but rather go through a number of intermediate carriers. How many complexes are involved from NADH to O2?
Electrons from NADH to O2 flow through three complexes (NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, and cytochrome c oxidase)
How do the electrons from from FADH2 to O2?
same as NADH but there is a fourth complex, succinate-Q reductase, which contains the enzyme succinate dehydrogenase from the citric acid cycle that generates FADH2. This is the point in the electron transport chain where the electrons from FADH2 enter.
iron is present in all of the complexes that form the electron transport chain; iron is a common electron carrier. Iron is present in which two forms?
it can be present in iron-sulfur clusters that are present in iron-sulfur proteins.
iron can be present in heme prosthetic groups that are part of proteins called cytochromes
The Respiratory Chain Consists of Proton Pumps and a Physical Link to the Citric Acid Cycle
Note that the location of the citric acid cycle in the mitochondrial matrix produces the NADH and FADH2 in the compartment where these two high-energy electron carriers can easily donate their electrons to the electron transport chain which is situated on the inside of the inner mitochondrial membrane.
the electrochemical gradient which generates a proton-motive force is the force or energy that is used to synthesize ATP
What is the enzyme complex that is responsible for using the proton gradient to generate ATP?
ATP Synthase
ATP Synthase consists of 2 components which are?
a F0 portion that is embedded in the inner mitochondrial membrane and contains a proton channel
an F1 component, which contains the catalytic activity
How does the binding change mechanism work?
the binding of protons that enter the “a” subunit causes the c ring to rotate, which in turn also causes rotation of the γ
and ε subunits. This rotation causes conformational changes of the a/β ring which is where ATP synthesis occurs (specifically the β-subunit).
What are the 3 functions of the active sites of the β-subunits in ATP synthase?
binding of ADP and Pi which are the substrates for ATP synthesis;
catalysis of ATP synthesis; and release of ATP
followed by binding of ADP and Pi.
At any given time, the three
β-subunits of ATP synthase are each engaged in one of these three different functions
What are the three different conformations that the β-subunit adopts?
the O (open), L (loose), and T (tight) conformations.
The O-form is an open conformation that can bind or release adenine nucleotides including ATP.
The L-form binds ADP and Pi, and
the T-form binds ATP very tightly, to the point where it will convert the bound ADP and Pi to ATP.
What causes the conformation of each
β-subunit to change?
It is the rotation of the
γ-subunit, which rotates when the c-ring rotates.
Once in the open conformation, the β-subunit can release its ATP and subsequently bind an ADP and Pi
An overview of oxidative phosphorylation, including the components of the electron transport chain, ATP synthase, and the proton gradient produced that is used to drive ATP synthesis, is shown
Note that protons are pumped into the intermembrane space, while ATP is formed in the mitochondrial matrix.
How do high-energy electrons from NADH produced in the cytosol get to the electron transport chain?
The Glycerol-3-P Shuttle
Explain the glycerol-3-P Shuttle
NADH passes its electrons to dihydroxyacetone phosphate, which becomes reduced to glycerol-3-P.
This can pass through the porous outer mitochondrial membrane into the intermembrane space, where mitochondrial glycerol-3-P dehydrogenase acts on it to oxidize it back to dihydroxyacetone phosphate.
FAD is a prosthetic group of this enzyme, and gets reduced to FADH2.
The production of FADH2 in the inner mitochondrial membrane facilitates the entry of these electrons into the electron transport chain through the reduction of coenzyme Q (an intermediate carrier).
How do electrons from NADH transfer into the mitochondrial matrix in the heart and liver?
The Malate-Aspartate Shuttle
How does the Malate-Aspartate Shuttle work?
A transporter in the inner mitochondrial membrane transports malate into the mitochondrial matrix in exchange for
α-ketoglutarate.
Malate is oxidized back to oxaloacetate, producing NADH which can enter the electron transport chain.
Oxaloacetate does not cross the inner mitochondrial membrane, so instead it undergoes a transamination reaction (in a later module) to form aspartate, which can cross the membrane.
The rest of the reactions are simply present to regenerate oxaloacetate in the cytosol so that it can bring more electrons from NADH into the mitochondria using this shuttle system.
Most of the ATP produced by oxidative phosphorylation in the mitochondrial matrix has to move to the cytosol and other cell compartments where it is needed.
Furthermore, ADP must be able to move back into the mitochondrial matrix in order to be phosphorylated to ATP.
Yet neither ADP nor ATP diffuse freely through the inner mitochondrial membrane.
How does this happen?
by a specialized carrier called ATP-ADP translocase
How does ATP-ADP translocase work?
a carrier which brings ATP from the mitochondrial matrix to the cytosol, and ADP from the cytosol to the mitochondrial matrix.
This translocase is present in very high concentrations in the inner mitochondrial membrane, not surprising given the large amount of ATP that is synthesized every day.
This ATP-ADP exchange is not without energy cost, and in fact about the 25% of the energy generated by electron transfer in the respiratory chain is used to drive this exchange.
It should be noted that there are a number of other shuttles/transporters in the inner mitochondrial membrane that allow the exchange of biomolecules and ions between the cytosol and mitochondria.
How is cellular respiration regulated?
Cellular respiration is defined as the collective activities of the citric acid cycle and oxidative phosphorylation.
It is ultimately regulated by the need for ATP
How many ATP are produced from every NADH that enters the electron transport chain?
About 2.5
How many ATP are produced from every FADH2 that enters the electron transport chain?
About 1.5
What is the net ATP yield from every molecule of glucose oxidized?
the net ATP yield is 30 per glucose molecule oxidized
26 of which come from oxidative phosphorylation.
This compares to only 2 ATP produced when glucose is metabolized anaerobically through glycolysis alone.
