E Subunits

ATP synthase actually consists of two principal complexes. The spheres observed in electron micrographs make up the Fj unit, which catalyzes ATP synthesis. These Fj spheres are attached to an integral membrane protein aggregate called the Fq unit. Fj consists of five polypeptide chains named a, j3, y, 8, and e, with a subunit stoichiometry ajjSaySe (Table 21.3). Fq consists of three hydrophobic subunits denoted by a, b, and c, with an apparent stoichiometry of ajbgCg.ig- Fq forms the transmembrane pore or channel through which protons move to drive ATP synthesis. The a, j3, y, 8, and e subunits of Fj contain 510, 482, 272, 146, and 50 amino acids, respectively, with a total molecular mass... 

FIGURE 21.25 A model of the Fj and Fg components of the ATP synthase, a rotating molecnlar motor. The a, b, a, /3, and 8 snbnnits constitute the stator of the motor, and the c, y, and e subunits form the rotor. Flow of protons through the structure turns the rotor and drives the cycle of conformational changes in a and fi that synthesize ATP. 

In 1984, the first methanogenic CODH was isolated from Methano-sarcina barkeri (138) and was shown to consist of a complex of the a and e subunits, with an apparent molecular mass of 232 kDa. Similar results have been found for other methanogenic CODH preparations (139-143). CODH activity must reside in the a subunit of the methanogenic enzyme, since the e subunit lacks cysteine residues and could... 

Figure 12-9. Mechanism of ATP production by ATP synthase. The enzyme compiex consists of an Fq sub-compiex which is a diskof "C" protein subunits. Attached is a y-subunit in the form of a "bentaxie." Protons passing through the disk of "C" units cause it and the attached y-subunit to rotate. The y-subunit fits inside the F, subcompiex of three a- and three (3-sub-units, which are fixed to the membrane and do not rotate. ADP and P are taken up sequentiaiiy by the (3-subunits to form ATP, which is expeiied as the rotating y-subunit squeezes each (3-subunit in turn. Thus, three ATP moiecuies are generated per revoiution. For ciarity, not aii the subunits that have been identified are shown—eg, the "axie" aiso contains an e-subunit.

Photoreactive aryl azide linked retinoic acid derivative (termed ADAM-3,55, Fig. 18) labeled efficiently the recombinant E-subunit and the covalent attachment was identified within residues 492 - 510 and 589 - 594, which correspond to similar sequences found in the human retinoic receptor [148]. 

The ATP synthase (EC3.6.1.34, complex V) that transports H"" is a complex molecular machine. The enzyme consists of two parts—a proton channel (Fq, for oligomycin-sensitive ) that is integrated into the membrane and a catalytic unit (Fi) that protrudes into the matrix. The Fo part consists of 12 membrane-spanning c-peptides and one a-subunit. The head of the Fi part is composed of three a and three p subunits, between which there are three active centers. The stem between Fo and Fi consists of one y and one e subunit. Two more polypeptides, b and 8, form a kind of stator, fixing the a and p subunits relative to the Fo part. 

Fig. 7. Extension of the sequence of events at a single p subunit to the ATP synthase enzyme as a whole during ATP synthesis, as proposed by the torsional mechanism. The diagram is drawn at 60° intervals of the movement of the e subunit. Binding of substrate converts the two-nucleotide state (the resting or ground state) to the three-nucleotide state. Catalysis takes place in the three-nucleotide state, which converts back to the two-nucleotide state with release of product [18]...

The e subunit plays a crucial role in release of product. 

The e subunit (in E. coli) is always closest to the catalytic site with the least affinity for ATP (Pe, defined only for the two-nucleotide state). 

Nicotinic ACh receptor (AChR) at the muscle end plate. A. The AChR is a pentameric complex made up of five subunits surrounding a central conducting channel. Embryonic AChR, containing the 7-subunit as shown, is a low-conducting channel. Adult AChR has instead an e-subunit and is a high-conducting channel. 

ACh receptors are present in the post-junctional membrane of the endplate, in the junctional folds. The nicotinic ACh receptor at the motor endplate has five subunits, two os, (3, 5 and . In addition, a Y subunit instead of an e subunit may be present in the so-called extra-junctional or the fetal receptor. The five subunits are arranged as a cylinder around a central funnel-shaped pore, the ion channel. The two a subunits each carry a recognition site which binds nicotinic agonists such as ACh and antagonists such as the neuromuscular blocking agents. Whilst ACh must bind to both subunits to produce an effect, it is sufficient for... 

What is known is that the ( -subunits from different species are able to form rings of between 10 and 14 proteolipids. For example, the x-ray crystal structure of the yeast F-ATPase (Stock et at, 1999) shows that there are 10 e-subunits, whereas the chloroplast enzyme has 14 (Seelert et at,... 

Capaldi, R. A., Aggeler, R., Gogol, E. P., and Wilkens, S. (1992). Structure of the Escherichia coli ATPsynthase and role of the y and e subunits in coupling catalytic sites and proton channeling functions. J. Bioenerg. Biomembr. 24, 435-439. 

Sambade, M., and Kane, P. M. (2004). The yeast V-ATPase contains a subunit homologous to the M. sexta and bovine e subunits that is essential for function./ Biol. Chem. 279, 17361-17365. 

Tsunoda, S. P., Rodgers, A. J., Aggeler, R., Wilce, M. C., Yoshida, M., and Capaldi, R. A. (2001b). Large conformational changes of the e subunit in the bacterial F F0 ATP synthase provide a ratchet action to regulate this rotary motor enzyme. Proc. Natl. Acad. Sci. USA 98, 6560-6564. 

Uhlin, U., Cox, G. B., and Guss, J. M. (1997). Crystal structure of the e subunit of the proton-translocating ATP synthase from Escherichia coli. Structure 15, 1219-1230. 

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