Secondary structural composition of the Na/K-ATPase E1 and E2 conformers

Abstract

The existence of conformers of the sodium- and potassium-dependent adenosine triphosphatase has been known for some time, yet their structures remain poorly characterized. In this study, circular dichroism spectroscopy was utilized to assess the secondary structural composition of the enzyme, particularly with regard to the E1 and E2 states that are associated with the presence of Na+ and K+, respectively. Parallel experiments were performed in which highly purified Na/K-ATPase from guinea pig kidney outer medulla was incubated with various cations and then examined by CD. The spectra were corrected for optical effects which arise due to the particulate nature of the membrane-bound protein, and then fit to reference data derived from a set of proteins with known secondary structures. In the peptide backbone region of the spectrum (190-240 nm), significant differences between the E1 and E2 conformers were detected and quantified in terms of the proportions of secondary structures present. An extensive conformational change rather than a small local perturbation must be responsible for the differences observed.