Water and cytochrome oxidation-reduction reactions

Abstract

Heavy water ( 2H 2O) substitution for H 2O in photosynthetic bacterial whole cells and mitochondrial membrane fragments results in an isotope effect of √2 for the ratio of the half-times [ t 1 2 ( 2 H 2 O) /t 1 2 ( H 2 O) ] of oxidation-reduction reactions involving cytochromes. This result, along with the absolute requirement for water, and the effects on the half-times of the substitution of hydrogen-bonding molecules such as ethylene glycol and glycerol for water suggest the direct participation of hydrogen atoms of water and water rotation in the rate-determining step of these electron transfer reactions. Arrhenius plots of cytochrome photooxidation half-times measured at various temperatures from 300 to 77 °K show the half-times to be temperature-dependent from 300 °K to about 150 °K and temperature-independent from about 150 °K through liquid N 2 temperatures. 2H 2O substitution does not change the qualitative response of half-times to temperature; however, the half-times are retarded by √2 throughout the entire temperature range investigated. If the √2 factor found in these experiments is indicative of proton or hydrogen atom motion, then direct electron transfer by electron tunnelling can not be the rate-limiting step in the cytochrome oxidation-reduction processes studied. The experimental results suggest that electron transfer between membrane-bound electron carrier molecules may be effected by means of hydrogen atom transfer via “water bridges” in biological oxidation-reduction reactions.