1. Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, A. N. Belozersky Insitute of Physico-Chemical Biology, Moscow State University, Moscow 119899, Russia, and Department of Physiology and Biophysics, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York, New York 10461