Chemiosmotic Theory of Mitochondrial Membrane | Biologic Oxidation

In this article we will discuss about the Chemiosmotic Theory of Mitochondrial Membrane.

It is explained that the primary event in oxidative phosphorylation is the translocation of protons (H⁺) to the exterior of a coupling mem­brane driven by oxidation in the respiratory chain.

The membrane is impermeable to protons which accumulate outside the membrane creating an electrochemical potential difference across the membrane. This electrochemical potential differ­ence is utilized to drive a membrane-located ATP synthetase which, in the presence of P_i+ A DP, forms ATP (Fig. 12.28).

It is suggested that the respiratory chain is folded into three oxidation/reduction (o/r) loops in the membrane, each loop functionally corre­sponds to site I, site II, and site III of the respiratory chain. A single loop consists of a hydrogen carrier and an electron carrier shown in Fig. 12.29.

A con­figuration of the respiratory chain folded into three functional o/r loops is shown in Fig. 12.30.

Each electron pair transferred from NADH to oxygen causes six protons to be trans-located from the inside to the outside of the mitochondrial mem­brane. NADH first donates one proton and two elec­trons, which, together with another proton from the internal medium, reduce FMN to FMNH₂. FMN is considered to extend the full width of the mem­brane and enables it to release two protons to the outside of the membrane and then to return two electrons to the inside surface via FeS proteins which is reduced.

Each reduced FeS complex do­nates one electron to an ubiquinone (Q) molecule which, upon taking up a proton from inside the membrane, forms QH₂. QH₂, being lipid-soluble and a small molecule, is free to move to the outside of the membrane where it discharges a proton pair into the cytosol and donates two electrons to two mol­ecules of the next carrier in the respiratory chain, cytochrome b.

This electron carrier is thought to span the mitochondrial membrane, enabling the electrons to join another molecule of ubiquinone together with two more protons from the internal medium. The resulting QH₂ shuttles to the outer surface where two protons are liberated and two electrons passed to two molecules of cytochrome C.

These electrons then pass through the remainder of the cytochrome chain, traversing the membrane to cytochrome aa₃ which lies on the inside of the membrane. At this site, two electrons combine with two H⁺ from the internal medium and an oxygen atom to form water.

The phosphorylating subunits responsible for the production of ATP are scattered over the sur­face of the inner membrane. These consist of sev­eral proteins collectively known as an F, subunit which projects into the matrix and which contains the ATP synthetase (Fig. 12.28).

These subunits are attached, possibly by the stalk, to a membrane protein subunit known as F₀, which probably ex­tends through the membrane. For every proton pair passing through the F₀ – F₁ complex, one ATP mol­ecule is formed from ADP and P_i.

A model was suggested which is shown in Fig. 12.31.

A proton pair attacks one oxygen of P_i to form H₂O and an active form of P_i which immediately combines with ADP to form ATP. Other studies have suggested that ATP synthesis is not the main energy-requiring step-rather it is the release of ATP from the active site. This may involve conforma­tional changes in the F₁ subunit.

The ion leakage would have to be balanced by extrusion of ions against the electric gradient to prevent swelling and lysis. It was, therefore, postu­lated that the coupling membrane contains ex­change diffusion systems for exchange of anions against OH⁻ ions and of cations against H⁺ ions. Such system would be necessary for uptake of ion­ized metabolites through the membrane.

The electrochemical potential difference across the membrane would inhibit further trans­port of reducing equivalents through the o/r loops unless it was discharged by back translocation of protons across the membrane through the vectorial ATP synthetase system. This depends on the avail­ability of ADP and P_i.

Several points arise from the chemiosmotic theory that have experi­mental support.

These are:

1. Mitochondria are generally impermeable to protons and other ions. The specific transport systems enable ions to penetrate the inner mitochon­drial membrane.

2. Un-couplers like dinitrophenol increase the permeability of mitochondria to protons reducing the electrochemical potential for the generation of ATP.

3. Addition of acid to the exter­nal medium leads to the gen­eration of ATP.

4. The P/H⁺ (transported out) quotient of the ATP synthetase is 1/2 and the H⁺/O quotients for succinate and β- hydroxybutyrate oxidation are 4 and 6, respectively, conforming with the expected P/O ratios of 2 and 3, respec­tively. These ratios are compatible with the postulated existence of o/r loops in the respiratory chain.

5. Oxidative phosphorylation does not oc­cur in soluble systems where there is no possibility of a vectorial ATP synthetase.

The Creatine phosphate shuttle facilitates transport of high energy phosphate from mitochon­dria:

1. An isoenzyme of creatine kinase is found in the mitochondria catalysing the trans­fer of high energy phosphate to creatine from ATP. The creatine phosphate is trans­ported into the cytosol via protein pores in the outer mitochondrial membrane.

2. Different isoenzymes of creatine kinase cause transfer of high-energy phosphate to and from the various systems that uti­lize it (e.g., muscle contraction, glycoly­sis).