The Problem with Inventing Molecular Mechanisms to Fit Thermodynamic Equations of Muscle

Abstract

Almost every model of muscle contraction in the literature to date is a molecular power stroke model, even though this corpuscular mechanism is opposed by centuries of science, by 85 years of unrefuted evidence that muscle is a thermodynamic system, and by a quarter century of direct observations that the molecular mechanism of muscle contraction is a molecular switch, not a molecular power stroke. An ensemble of molecular switches is a binary mechanical thermodynamic system from which A.V. Hill’s muscle force–velocity relationship is directly derived, where Hill’s parameter a is the internal force against which unloaded muscle shortens, and Hill’s parameter b is the product of the switch displacement, d, and the actin–myosin ATPase rate. Ignoring this model and the centuries of thermodynamics that preceded it, corpuscularians continue to develop molecular power stroke models, adding to their 65-year jumble of “new”, “innovative”, and “unconventional” molecular mechanisms for Hill’s a and b parameters, none of which resemble the underlying physical chemistry. Remarkably, the corpuscularian community holds the thermodynamicist to account for these discrepancies, which, as outlined here, I have done for 25 years. It is long past time for corpuscularians to be held accountable for their mechanisms, which by all accounts have no foundation in science. The stakes are high. Molecular power stroke models are widely used in research and in clinical decision-making and have, for over half a century, muddied our understanding of the inner workings of one of the most efficient and clean-burning machines on the planet. It is problematic that corpuscularians present these models to stakeholders as science when in fact corpuscularians have been actively defending these models against science for decades. The path forward for scientists is to stop baseless rejections of muscle thermodynamics and to begin testing corpuscular and thermodynamic mechanisms with the goal of disproving one or the other of these hypotheses.

Within Thermal Scales: The Kinetic and Energetic Pull of Chemical Entropy

“…Clearly this analysis does not apply to corpuscular models of chemical mechanics in which the system term, k B T·lnΩ i , is arbitrarily removed from the free energy equation for the sole purpose of making the energy of the system a simple sum of the free energy of its molecular (corpuscular) parts, or G i = Gº i . While corpuscular models have a molecularly explicit appeal, they are non-physical [8] and clearly have little relevance to the question that is the focus of this physical chemical analysis. Is Ω i a chemical activity of or the number of microstates within state i.…”