Question

A representation of the structure of the mitochondrial ATP synthase is presented in Figure 2 of this document. Notably, the chloroplast version of the spinach ATP synthase is remarkably similar to the mitochondrial version of the ATP synthase, with one notable exception. In the Fo portion of the chloroplast ATP synthase there are 14 "c" subunits. Discuss this noteworthy structural difference in the ATP synthases relative to the values you calculated in parts D and E of this problem. F)

Answer 1

Ans: The ATP synthase enzyme in the course of evolution has remained conserved which is beneficial in that its mechanism somewhat follows the same whether in mitochondria or chloroplast, but still there remain subtle differences in their structure.
The chloroplast f₀ complex is composed of a,b & c subunits which represent a motor like structure whose rotation causes the catalytic part (the f₁ complex) to rotate and this brings about the formation of ATP.
The f₀ complex has 3 major sub-units a,b and c and the 'c' sub-unit has further subsections that depend on species to species. The ring from S. cerevisiae contains 10 subunits, the ring from I. tartaricus contains 11 subunits & the ring from spinach S. oleracea contains 14 subunits. The stoichiometry is important because it is directly related to the proton-to-ATP ratio and the proton-motive-force (p.m.f.) required for each proton to drive the enzyme and considering 100 % efficiency that each full rotation brings about 3 ATP molecules, so the more sub-units the more protons and less p.m.f it requires.

The mitochondrial f₀ complex has 3 major sub-units a,b,c and the 'c' sub-unit has further subsections containing 8 sub-units (as shown in bovine mitochondria), d, f₆ and the oligomycin sensitivity-conferring protein (OSCP). The mechanism is similar and that the protonation and deprotonation of specific residues bring about the rotation of these subsections that in turn rotates the f₁ complex.

The reason behind these structural differences is not clearly explained but it must be somehow related to the functional differences of the organelles.

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