A proton motive force transducer and its role in proton pumps, proton engines, tobacco mosaic virus assembly and hemoglobin allosterism

In this paper I am proposing a new, conformationally dependent basic site in proteins. The initial formulation of this proposal was based on the following: (1) bacteriorhodopsin is a light-driven proton pump and as such is a prototype for understanding proton-mediated energy transduction in biological systems; (2) current evidence suggests about 2 protons are pumped for each photon absorbed; (3) given the usual role of prolines as helix breakers, it is surprising to find about 2 prolines deeply embedded in the membrane-spanning, probably α-helical, portion of the bacteriorhodopsin molecule; (4) another presumptive proton translocator, the F₀ proteolipid, is also helical and has a critical proline in its structure; (5) workers interested in protein folding have explained the existence of fast and slow folding subgroups of the same protein molecule as being due to cis : trans isomerization about the proline imide group; (6) the cis : trans isomerization is acid catalyzed; (7) simple chemical considerations predict that the proton affinity of the proline nitrogen should increase dramatically as the imide group is distorted away from planarity and should be a maximum midway between the cis and trans forms; thus, stabilization of the intermediate by protonation accounts for the acid catalysis of the proline cis : trans isomerization.Linking these observations together suggests that proline-containing α-helices may play a role in proton motive energy transduction. Due to the absence of a proton on the proline nitrogen, a proline-containing helix has a “proton hole” between the proline nitrogen and the carbonyl oxygen four residues earlier in the sequence. Here I propose a model in which the paramount feature is the change in pK_a associated with a change in geometry of the “proton hole.” Order of magnitude calculations suggest that the proton hole should change its pKa by about 8 units, corresponding to a 10⁸ change in proton affinity, for every 10 kcal of distortion energy, V. Calculations also show that it is energetically feasible to modulate the pK_a of this site over the dynamic range of pK_a = 2–14. Such a large value for $\text{ΔpK}{}_{\mathrm{a}}\text{ΔV}$ and such a dynamic range makes this site an ideal basis for an “integral proton injector,” an abstract model for proton pumping suggested on purely theoretical grounds by Nagle &; Mille (J. chem. Phys.74, 1367–1372, 1981).Finally, two well studied proteins, the α-chain of hemoglobin and tobacco mossaic virus coat protein, both show features in their X-ray determined structures suggesting the possibility of protonation and deprotonation of the proton hole in α-helices containing proline. For TMV coat protein, there is a proline-containing α-helix that is located precisely in the region of the protein which undergoes an acid-induced conformational rearrangement. Structural changes at this locus have been singled out in comparisons of the X-ray structures of the TMV protein in its two conformations. For the α-chain of horse hemoglobin, there are two concurrent sites that are likely protonated and one contrary site that likely becomes deprotonated as hemoglobin converts from the liganded to the deoxy form. The contrary proline is proposed to help maintain co-operative oxygen binding over a wide pH range. The absence of one of the concurrent proline site in marsupial hemoglobin accounts for the small Bohr effect exhibited by these hemoglobins. The absence of the contrary proline site in carp hemoglobin accounts in a very logical way for the large Bohr effect and the lack of cooperative oxygen binding at both low and high pH by this hemoglobin.