In this article we will discuss about:- 1. Definition of Oxidative Phosphorylation 2. Chemiosmotic Hypothesis and Oxidative Phosphorylation 3. Inhibition.
Definition of Oxidative Phosphorylation:
Oxidative phosphorylation is the process by which energy from electron transport chain (respiratory chain) is used to make ATP, and is the culmination of energy yielding metabolism in aerobic organisms. Oxidative phosphorylation involves the reduction of O2 to H2O with electrons donated by NADH and FADH2, and equally occurs in light or darkness.
Our current understanding of ATP synthesis is based on chemiosmotic hypothesis first formulated in 1961 by Peter Mitchell, a British biochemist who later received the Nobel Prize for this important contribution. Chemiosmotic hypothesis has been accepted as one of the great unifying principles of 20th century biology.
It provides insight into not only the processes of photophosphorylation and oxidative phosphorylation but also the processes of disparate energy transductions as active transport across membranes and the rotation of bacterial flagella.
Chemiosmotic Hypothesis and Oxidative Phosphorylation:
According to chemiosmotic hypothesis the electron transport chain is organized so that protons move outward from the mitochondrial matrix to inter-membrane space (in eukaryotes; Fig. 24.6) and from cytoplasm to periplasmic space passing across the plasma membrane (in prokaryotes; Fig. 24.7).
Proton movement may result either from different complexes or from the action of special proton pumps that derive their energy from electron transport resulting in proton motive force (PMF) composed of a gradient of protons and a membrane potential due to the unequal distribution of charges.
1. Generation of Proton Motive Force (PMF):
When O2 is reduced to H2O after accepting electrons transferred from electron transport chain, it requires proton (H+) from the cytoplasm to complete the reaction.
These protons originate from the dissociation of water into H+ and OH–. The use of H+ in the reduction of O2 to H2O and the extrusion of H+ outside the membrane during electron transport chain (Fig. 24.8) cause a net accumulation of OH– on the inside of the membrane.
Despite their small size, because they are charged, neither H+ nor OH– freely passes through the membrane, and so equilibrium cannot be spontaneously restored on both sides of membrane.
This non-equilibrium state of H+ and OH– on opposite sides of the membrane results in the generation of a pH gradient and an electrochemical potential across the membrane, with the inside of the membrane (cytoplasm side) electrically negative and alkaline, and the outside of the membrane electrically positive and acidic.
This pH gradient and electrochemical potential cause the membrane to be energised. The energised state of a membrane, which is referred to as proton motive force (PMF) and is expressed in volts, is used directly to drive the formation of ATP, ion transport, flagellar rotation, and other useful work.
2. Proton Motive Force and ATP Synthesis:
Proton motive force-derived ATP synthesis involves a catalyst, which is a large membrane enzyme complex called ATP synthase or ATPase for short (Fig. 24.9).
The ATPase contains two major parts:
(1) A multi-subunit head piece called F1 located on mitochondrial matrix side (in eukaryotes) and on cytoplasmic side (in prokaryotes) and
(2) A proton conducting channel called F0 that resides in the inner membrane of mitochondrion (in eukaryotes) and in plasma membrane (in prokaryotes) and spans the membrane.
The ATP synthesis takes place at the F1/F0 ATPase, which is the smallest known biological motor. F1, is the catalytic complex responsible for the inter conversion of ADP + Pi (inorganic phosphate) and ATP, and consists of five different polypeptides present as an α3 β3 ϒƐδ complex. F0 is integrated in the membrane and consists of three polypeptides in an ab2 c12 complex. 3, 3, 2 and 12 denote the numerical numbers of α, β, b and c, respectively.
According to the current model of how the ATPase functions in Escherichia coli (Fig. 24.9), subunit ‘a’ is responsible for channeling protons (H+ ) across the membrane while subunit b protrudes outside the membrane and forms, along with b2 and δ subunit, the stator. Protein movement through ‘a’ submit of F0 drives rotation of the c proteins generating a torque that is transmitted to F1, by the ϒε subunits.
As a result, energy is transferred to F1through coupled rotation of yε subunits causing conformational changes in the β subunits. The conformational changes in the β subunits allow for binding of ADP + Pi and these are converted to ATP when the β subunits return to their original conformation.
Inhibition of Oxidative Phosphorylation (ATP Synthesis):
Many chemicals inhibit the synthesis of ATP and can even kill cells to sufficiently high concentrations. Two such classes of chemicals are known inhibitors and un-couplers. Inhibitors directly block electron transport chain.
The antibiotic piericidin competes with coenzyme Q; the antibiotic antimycin. A blocks electron transport between cytochromes b, and c, and both carbon monoxide (CO) and cyanide (CN–) bind tightly to certain cytochromes and prevent their functioning.
The un-couplers, in contrast, prevent ATP synthesis without affecting electron transport chain itself. Normally the electron transport chain is tightly coupled with oxidative phosphorylation, and the un-couplers disconnect oxidative phosphorylation from electron transport chain.
Therefore, the energy released by the chain is given off as heat rather than as ATP. Many lipid soluble un-couplers (e.g., dinitrophenol, dicumarol, valinomycin) make membranes leaky allowing free passage of protons through the membrane without activating F1/F0 ATPase. In this way they destroy the proton motive force and its ability to drive ATP synthesis.