Electron self-exchange in hemoglobins revealed by deutero-hemin substitution

• Electron self-exchange rate constants were measured for several hemoglobins.
• Measurements used deutero- and proto-heme hemoglobins in different oxidation states.
• These rate constants are consistent with those measured by chemical exchange NMR.
• Electron exchange is much faster for class 1 rice nonsymbiotic hemoglobin than for myoglobin.
• Electron exchange is consistent with rate constants for hydroxylamine reduction.

Hemoglobins (phytoglobins) from rice plants (nsHb1) and from the cyanobacterium Synechocystis (PCC 6803) (SynHb) can reduce hydroxylamine with two electrons to form ammonium. The reaction requires intermolecular electron transfer between protein molecules, and rapid electron self-exchange might play a role in distinguishing these hemoglobins from others with slower reaction rates, such as myoglobin. A relatively rapid electron self-exchange rate constant has been measured for SynHb by NMR, but the rate constant for myoglobin is equivocal and a value for nsHb1 has not yet been measured. Here we report electron self-exchange rate constants for nsHb1 and Mb as a test of their role in hydroxylamine reduction. These proteins are not suitable for analysis by NMR ZZ exchange, so a method was developed that uses cross-reactions between each hemoglobin and its deutero-hemin substituted counterpart. The resulting electron transfer is between identical proteins with low driving forces and thus closely approximates true electron self-exchange. The reactions can be monitored spectrally due to the distinct spectra of the prosthetic groups, and from this electron self-exchange rate constants of 880 (SynHb), 2900 (nsHb1), and 0.05 M− 1 s− 1 (Mb) have been measured for each hemoglobin. Calculations of cross-reactions using these values accurately predict hydroxylamine reduction rates for each protein, suggesting that electron self-exchange plays an important role in the reaction.

Electron self-exchange was measured for several hemoglobins using a method involving the exchange of proto-hemin IX for deutero-hemin IX. The resulting rate constants are consistent with those measured by chemical exchange NMR, and demonstrate that self-exchange in plant hemoglobin is fast enough to support reduction of hydroxylamine to ammonium.Figure optionsDownload as PowerPoint slide