Oxidative Phosphorylation* Flashcards
Know that a natural uncoupler protein can be activated to ‘short-circuit’ mitochondrial energy coupling for the purpose of producing heat (thermogenesis), and that a natural inhibitor protein can inactivate the ATP synthase under conditions where it would function to rapidly hydrolyze cellular ATP (ischemia).
Uncouplers uncouple oxidation from phosphorylation. The effect of uncouplers on respiratory control. Uncouplers stimulate oxygen consumption in the absence of ADP.
Uncouplers dissipate the proton gradient by providing an alternate route for proton re- entry into the matrix. In the absence of a gradient, the synthase runs in reverse hydrolyzing ATP.
Uncouplers do not need transporters. Uncouplers bind protons & bring them inside & lessen the gradient, they then go back out & keep doing the same thing. Uncouplers are energetically favorable. If you dissipate the gradient, it hydrolyzes the ATP! The ATP synthase becomes an ATPase.
Properties of uncouplers: weak acids, hydrophobic, delocalized charge
Dinitrophenol is an uncoupler
Uncouplers can be fatal, but the uncoupler protein is natural. It allows protons to leak down their electrochem gradient via its proton pore. It is under hormonal control norepi turns it on & it is in brown fat by activating lipases. Non-shivering thermogenisis occurs in hibernating mammals.
It is used by fetuses.
If we turn this natural uncoupler on, it can be used for fat loss.
A natural phosphorylation inhibitor (the inhibitor protein) protects against rapid ATP hydrolysis during ischemia. Abundant in heart tissue. Under normal conditions it does not bind to FoF1. But in the absence of O2, the pH of the matrix space drops and the inhibitor protein is protonated changing its conformation to a form that binds tightly to FoF1. When O2 is reintroduced, protons are pumped out of the matrix, the inhibitor protein is deprotonated and it pops off to allow resumption of ATP synthesis.
In ischaemic event, no ATP is being made because no O2. Therefore it becomes an ATPase (hydrolyzing all ATP in cells). This process hastens the death of tissue–therefore, the inhibitor protein inhibits this in the acidic enviornment (since non are being brought in, they build up). This protein is only on during emergency. When you get O2 back into the tissue, the inhibitor protein loses its affinity for ATP synthase.
Understand the concept of energy coupling in oxidative phosphorylation; the unique role of the transmembrane, electrochemical gradient as a ‘common intermediate’ linking oxidation/reduction reactions to ATP synthesis, and other membrane transport processes.
The basic premise of the chemiosmotic theory is that a delocalized electrochemical gradient is a required intermediate in coupling the exergonic redox reactions to the endergonic synthesis of ATP.
This is another example of the “Common Intermediate Principle”. The mechanism Mitchell proposed in 1961 predicted that membranes are impermeable to protons and that an electrochemical gradient exists.
Know that there are two components of the electrochemical gradient: an _____ gradient and a ______ gradient.
Know that there are two components of the electrochemical gradient: an electrical gradient and a proton gradient.
Understand the basis for the Q cycle for direct pumping of protons.
The Q cycle (named for CoQ10) describes a series of reactions that describe how the sequential oxidation and reduction of the lipid-loving electron carrier, Coenzyme Q10 or Q.
The net result is that two protons have been transported out of the mitochondrion against an electrochemical gradient. Since only one electron flows down to oxygen (the other is recycled), the stoichiometry is 2H+/e- or 4H+/2 e-.
Understand the Binding Change Mechanism for ATP synthesis.
In step 1, the asymmetric γ subunit rotates 120° clockwise driving conformational changes in the three catalytic sites that alter their affinities for substrates and product. In step 2, ATP forms spontaneously from tightly bound ADP and Pi.
This energy comes from protons going down their electrochem gradient & drives the rotation of the motor.
(1) Energization is not required to make ATP at the catalytic site but rather to promote its release
from the site.
(2) The tight binding of substrate and product release occur simultaneously on separate but
interacting sites.
(3) The coupling of H+ transport to the binding changes requires rotation of subunits.
A H+ enters a channel from the side of the membrane having a high concentration of protons and moves to the middle of the membrane. From there it hops onto one of the c subunits. The complex of c’s then rotates until the proton is brought into alignment with a second channel where the proton completes its journey. Hence, when the c subunits rotate relative to subunit-a, γ is forced to rotate relative to the catalytic subunits. The drive is protons moving down an electrochemical gradient. The rotation of γεcn (n = 8 in mammals) obligatorily couples the exergonic and endergonic processes.
The FoF1-ATP synthase is now recognized to be a tiny molecular motor. Instead of being driven by a current of electrons, as are the motors we’re familiar with, the synthase is driven by a current of protons. 3 protons driven down their gradient, make 1 ATP.
12 protons are transported/ 1 NADH
Know that many secondary transporters are needed to shuttle metabolites in/out of mitochondria, and that some require energy input from the electrochemical gradient; know an example for electrogenic transport (driven by the electrical gradient) and for electroneutral transport (driven by the proton gradient).
Glycerol phosphate shuttle (~2 ATP/2 e-), present in some muscle and nerve cells.
Doesn’t involve membrane transport since the mitochondrial glycerol-phosphate dehydrogenase is on the outside surface of the inner membrane.
Malate/aspartate shuttle (~3 ATP/2 e-) present in liver and heart. This one does involve membrane transport.
