Thermodynamics and Kinetics of Cytochrome C Oxidase and Bacteriorhodopsin

Hendler, R

Abstract

A new ultrarapid scan spectrometer was used to collect entire spectra with a time resolution of 10 micros, for the first time. Two energy transducing proteins were studied, cytochrome aa3 (cytox) and bacteriorhodopsin (BR). Two new global procedures for spectral analyses were developed, based on the pseudoinverse operator of linear algebra, that allowed a distinction between cytochromes a and a3 of cytox. The kinetic constant for the transfer of the first electron from cytochrome c was about 220 per s. This electron was shared equally between heme a and CuA. In the resting enzyme, the second electron was added slowly (k=about 0.05 per s). Reduction stopped at the 2 electron stage. Small amounts of O2 converted the enzyme to the pulsed state and allowed the full 4 electron reduction. The kinetic rate for internal electron transfer from cytochrome a to a3 increased from about 0.05 per s to about 100 per s. Time resolved kinetic spectra for the BR photocycle were accumulated over a 1000-fold range of laser activation light intensities, and analyzed by single value decomposition. It was found that at low light intensities, a photocycle occurred with a kinetically fast form of the M intermediate (Mf) which decayed directly to the O intermediate. At high light intensities, a different photocycle predominated with a slow form of the M intermediate (Ms) which decayed directly back to BR, without going through the O intermediate. The possibility that cooperative interactions of photons impinging on the fundamental trimer unit of BR is solely responsible for the phenomenon was eliminated by the finding that low levels of Triton destroy the """"""""cooperativity"""""""" before the trimer structure is decomposed to monomers. An essential involvement of membrane phospholipid is indicated.