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The effects of various mechanisms of metalloporphyrin reduction potential modulation were investigated experimentally using a robust, well-characterized heme protein maquette, synthetic protein scaffold H10A24 [¿CH(3)()CONH-CGGGELWKL.HEELLKK.FEELLKL.AEERLKK. L-CONH(2)()¿(2)](2). Removal of the iron porphyrin macrocycle from the high dielectric aqueous environment and sequestration within the hydrophobic core of the H10A24 maquette raises the equilibrium reduction midpoint potential by 36-138 mV depending on the hydrophobicity of the metalloporphyrin structure. By incorporating various natural and synthetic metalloporphyrins into a single protein scaffold, we demonstrate a 300-mV range in reduction potential modulation due to the electron-donating/withdrawing character of the peripheral macrocycle substituents. Solution pH is used to modulate the metalloporphyrin reduction potential by 160 mV, regardless of the macrocycle architecture, by controlling the protonation state of the glutamate involved in partial charge compensation of the ferric heme. Attempts to control the reduction potential by inserting charged amino acids into the hydrophobic core at close proximity to the metalloporphyrin lead to varied success, with H10A24-L13E lowering the E(m8.5) by 40 mV, H10A24-E11Q raising it by 50 mV, and H10A24-L13R remaining surprisingly unaltered. Modifying the charge of the adjacent metalloporphyrin, +1 for iron(III) protoporphyrin IX or neutral for zinc(II) protoporphyrin IX resulted in a loss of 70 mV [Fe(III)PPIX](+) - [Fe(III)PPIX](+) interaction observed in maquettes. Using these factors in combination, we illustrate a 435-mV variation of the metalloporphyrin reduction midpoint potential in a simple heme maquette relative to the about 800-mV range observed for natural cytochromes. Comparison between the reduction potentials of the heme maquettes and other de novo designed heme proteins reveals global trends in the E(m) values of synthetic cytochromes.

Proton exchange with aqueous media coupled to heme oxidation/reduction is commonly seen but not understood in natural cytochromes. Our synthetic tetrahelix bundle heme protein maquettes successfully reproduce natural proton coupling to heme oxidation/reduction. Potentiometry reveals major pK shifts from 4.2 to 7.0 and from 9.4 to 10.3 in the maquette-associated acid/base group(s) upon heme reduction. Consequently, a 210 mV decrease in the heme redox potential is observed between the two extremes of pH. Potentiometry with resonance Raman and FTIR spectroscopy performed over a wide pH range strongly implicates glutamate side chains as the source of proton coupling below pH 8.0, whereas lysine side chains are suggested above pH 8.0. Remarkably, the pK values of several glutamates in the maquette are elevated from their solution value (4.4) to values as high as 7.0. It is suggested that these glutamates are recruited into the interior of the bundle as part of a structural rearrangement that occurs upon heme binding. Glutamate to glutamine variants of the prototype protein demonstrate that removal of the glutamate closest to the heme diminishes but does not abolish proton exchange. It is necessary to remove additional glutamates before pH-independent heme oxidation/reduction profiles are achieved. The mechanism of redox-linked proton coupling appears to be rooted in distributed partial charge compensation, the magnitude of which is governed by the dielectric distance between the ferric heme and acid/base side chains. A similar mechanism is likely to exist in native redox proteins which undergo charge change upon cofactor oxidation/reduction.