Question

Explain the different theories of oxidative phosphorylation.10 marks​

Answer 1

Answer:

The following points highlight the top one classical theories of oxidative phosphorylation. The theories are: 1. The Chemical Coupling Theory

Classical Theory # 1. The Chemical Coupling Theory:

This theory was first proposed by Slater in 1953 and is based on the principles of sub­strate-level phosphorylation as illustrated by reaction sequence for glyceraldehyde-3-phosphate dehydrogenase in glycolysis resulting in the formation of one ATP molecule and 3-phosphoglyceric acid, (For details see glycolysis).

According to this theory (Fig. 16.13) a reduced electron carrier of the respiratory chain (e.g., AH2) reacts with an oxidised carrier (e.g., B) which is adjacent to it with sufficient free energy drop occurring to allow the reaction of A with an unknown compound C to give a non phosphorylated high energy intermediate compound A~C. B is reduced to BH2.

 

The following points highlight the top three classical theories of oxidative phosphorylation. The theories are: 1. The Chemical Coupling Theory 2. The Conformational Coupling Theory and 3. The Chemiosmotic Coupling Theory.

Classical Theory # 1. The Chemical Coupling Theory:

This theory was first proposed by Slater in 1953 and is based on the principles of sub­strate-level phosphorylation as illustrated by reaction sequence for glyceraldehyde-3-phosphate dehydrogenase in glycolysis resulting in the formation of one ATP molecule and 3-phosphoglyceric acid, (For details see glycolysis).

According to this theory (Fig. 16.13) a reduced electron carrier of the respiratory chain (e.g., AH2) reacts with an oxidised carrier (e.g., B) which is adjacent to it with sufficient free energy drop occurring to allow the reaction of A with an unknown compound C to give a non phosphorylated high energy intermediate compound A~C. B is reduced to BH2.

 

In the next sequential exchange reactions, C is transferred to phosphate to form a phosphorylated intermediate C~P. The electron carrier A becomes free and oxidised.

Finally, phosphate from C~P is transferred to ADP to give ATP. The unknown compound C becomes free and recycled.

Alternatively, there may be a parallel scheme of reaction in which the high-energy non- phosphorylated intermediate is formed with BH2 instead A

The chemical coupling theory did not find much support because no phosphorylated in­termediates or high-energy intermediates of the respiratory carriers have yet been identified unequivocally.