X-ray experiment provides a way to reveal the distinction between discrete and continuous conformation of myosin head

TL;DR

This paper proposes an X-ray experimental method to distinguish whether myosin heads adopt discrete states or a continuous conformation during muscle contraction, providing a potential way to verify a fundamental assumption in muscle biophysics.

Contribution

It introduces a theoretical framework for analyzing X-ray patterns to experimentally differentiate between discrete and continuous myosin head conformations.

Findings

X-ray pattern differences depend on conformation type.

Discrete states cause additional interference in X-ray signals.

Method could identify conformational states with high-sensitive equipment.

Abstract

The corner stone of the classical model after Huxley and Simmons is supposition that a myosin head can reside only in several discrete states and irregularly jumps from one state to another. Until now, it has not been found a way to experimentally verify this supposition although confirmation or refutation of the existence of discrete states is crucial for the solution of myosin motor problem. Here I show that a set of equal myosin heads arranged equidistantly along an actin filament produce X-ray pattern which varies with the type of conformation. If the lever arms of all myosin heads reside in one and the same position (continuous conformation), all the heads have the same form-factor and equally scatter electromagnetic wave. In this case, only the geometric factor associated with a spatial ordering of the heads will determine the X-ray pattern. The situation changes if the average lever arm position is the same, but inherently every head can reside only in several diverse discrete states, hopping irregularly from one to another. In this case, the form-factors corresponding to distinct states are dissimilar, and the increments in phases of X-rays scattered by different heads are different as well. Inasmuch as every quantum of radiation interacts with the heads residing in different states, this results in additional interference and some peaks in the X-ray pattern should slacken or even extinct as compared with the pattern from the heads with the continuous-type conformation. The formulas describing both cases are compared in this article. In general, the distinction between X-ray patterns is insignificant, but they could be appreciably different at some stages of conformation process (respective lever arm position depends on the type of discrete model). Consequently, one can with luck attempt to find out this difference using a high-sensitive equipment.