(Figure 1-8). In this model, the mitochondrial subunit 8 contributes to a third membrane

spanning region, and a combination of subunit d and subunit F6 forms the hydrophilic
domain.

 The Fo subunit known as the a subunit in mitochondrial F1Fo ATP synthase has no

counterpart in the bacterial or chloroplast enzymes. It is a small polypeptide (50 amino

acids) folded into a helix-loop-helix. It is believed to play a role in the stabilization of the

central stalk and its absence in the bacterial and chloroplast enzymes may explain why


b2


Domains




F1 binding



Dimerization


Tether

Membrane


Bacteria


Mitochondri on


Figure 1-8. Speculative models for the b-like subunits. Shown are models for the
 bacterial, chloroplast and mitochondrial b subunits. The membrane-spanning
 regions are indicated by the black lines. An abundance of evidence supports
 the parallel arrangement of the bacterial b2 homodimer and the chloroplast bb '
 heterodimer. The mitochondrial analogue of the b subunit is believed to
 consist of up to four polypeptides. The model shown for the mitochondrial
 b8dF6 Structure is highly speculative.


bb'


b8dF,


Chloroplast