Proof of principle in a de novo designed protein maquette: an allosterically regulated, charge-activated conformational switch in a tetra-alpha-helix bundle.

New understanding of the engineering and allosteric regulation of natural protein conformational switches (such as those that couple chemical and ionic signals, mechanical force, and electro/chemical free energy for biochemical activation, catalysis, and motion) can be derived from simple de novo designed synthetic protein models (maquettes). We demonstrate proof of principle of both reversible switch action and allosteric regulation in a tetra-alpha-helical bundle protein composed of two identical di-helical subunits containing heme coordinated at a specific position close to the disulfide loop region. Individual bundles assume one of two switch states related by large-scale mechanical changes: a syn-topology (helices of the different subunits parallel) or anti-topology (helices antiparallel). Both the spectral properties of a coproporphyrin probe appended to the loop region and the distance-dependent redox interaction between the hemes identify the topologies. Beginning from a syn-topology, introduction of ferric heme in each subunit (either binding or redox change) shifts the topological balance by 25-50-fold (1.9-2.3 kcal/mol) to an anti-dominance. Charge repulsion between the two internal cationic ferric hemes drives the syn- to anti-switch, as demonstrated in two ways. When fixed in the syn-topology, the second ferric heme binding is 25-80-fold (1.9-2.6 kcal/mol) weaker than the first, and adjacent heme redox potentials are split by 80 mV (1.85 kcal/mol), values that energetically match the shift in topological balance. Allosteric and cooperative regulation of the switch by ionic strength exploits the shielded charge interactions between the two hemes and the exposed, cooperative interactions between the coproporphyrin carboxylates.

Self-assembly of heme A and heme B in a designed four-helix bundle: implications for a cytochrome c oxidase maquette.

Heme A, a prosthetic group of cytochrome c oxidase [EC 1.9.3.1], has been introduced into two de novo designed four helix bundle proteins, [H10A24](2) and [H10H24](2), known to bind 2-4 equiv of heme B, respectively [Robertson, D. E., Farid, R. S., Moser, C. C., Mulholland, S. E., Pidikiti, R., Lear, J. D., Wand, A., J., DeGrado, W. F., and Dutton, P. L. (1994) Nature 368, 425-432]. [H10A24](2), [Ac-CGGGELWKL x HEELLKK x FEELLKL x AEERLKK x L-CONH(2)](2)(2), binds two heme A molecules per four-helix unit via bis-histidine ligation at the 10,10' positions with measured K(d) values of <0.1 and 5 nM, values much lower than those measured for heme B (K(d) values of 50 and 800 nM). The heme A-protein complex, [heme A-H10A24](2), exhibits well-defined absorption spectra in both the ferric and ferrous states, and an electron paramagnetic resonance spectrum characteristic of a low spin heme in the ferric form. A single midpoint redox potential (E(m8)) was determined for [heme A-H10A24](2) at -45 mV (vs SHE), which is significantly higher than that of the protein bound heme B (-130 and -200 mV). The observation of a single midpoint redox potential for [heme A-H10A24](2) and a pair of midpoints for [heme B-H10A24](2) indicates that the di-alpha-helical monomers are oriented in an anti topology (disulfides on opposite sides of bundle) in the former (lacking heme-heme electrostatic interaction) and syn in the latter. A mixture of global topologies was indicated by the potentiometric titration of the related [heme A-H10H24](2) which possess two distinct reduction potentials of +41 (31%) and -65 mV (69%). Self-assembly of the mixed cofactor heme A-heme B-[H10A24](2) was accomplished by addition of a single equivalent of each heme A and heme B to [H10A24](2). The single midpoint redox potential of heme B, E(m8) = -200 mV, together with the split midpoint redox potential of heme A in heme A-heme B-[H10A24](2), E(m8) = +28 mV (33%) and -65 mV (67%), indicated the existence of both syn and anti topologies of the two di-alpha-helical monomers in this four helix bundle. Synthesis of the mixed cofactor [heme A-heme B-H10H24](2) was accomplished by addition of a 2 equiv of each heme A and heme B to [H10H24](2) and potentiometry indicated the pair of hemes B resided in the 10,10' sites and heme A occupied the 24,24' sites. The results indicate that heme peripheral structure controls the orientation of the di-alpha-helical monomers in the four-helix bundle which are interchangeable between syn and anti topologies. In the reduced form, [heme A-H10A24](2), reacts quantitatively to form [carbonmonoxy-heme A-H10A24](2) as evidenced by optical spectroscopy. The synthetic [heme A-H10A24](2) can be enzymatically reduced by NAD(P)H with natural reductases under anaerobic conditions, and reversibly oxidized by dioxygen to the ferric form.