FUNCTIONAL LABELING OF CYTOCHROME C BY H EXCHANGE & NMR

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It is generally recognized that the H-exchange behavior of the amide NH of a protein can yield information about structure, structure change, and structural dynamics all through the protein, but this capability has not yet been achieved. We have developed differential H-exchange methods that can label just those parts of any protein that participate in any interaction (allosteric change, small or large molecule binding, etc.). Recent advances in NMR now make it probable that one will be able to recognize many of the amide NH resonances in a protein like cytochrome c, and measure their individual exchange rates. The coupling of the HX and NMR capabilities would provide a most powerful tool for protein chemical studies. We propose to begin to develop this capability. The selective labelling methods will be used, with samples of fully ND-exchanged protein, to put H on just those segments of cytochrome c that act to adjust the redox potential of the heme, or that interact with the oxidase, or the reductase, etc. Then we will attempt to identify the sensitive (protonated) resonances using 2D NMR techniques supplemented by our selective tritium-labelling and peptide HPLC separation methods. We further suspect that measurement of the H-exchange rates of restricted sets of NH like these will distinguish between alternative models for the internal dynamical motions suggested to determine protein H-exchange processes. If the local unfolding model is confirmed, as now seems likely (at least for a-helices), then experiments like these will also be able to quantify the contribution of individual segments to protein function in terms of free energy. We hope to check this capability by comparing redox sensitive H-exchange changes in related cytochromes having differing redox potentials.