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In this paper, we present a refined version of the previously proposed suspension contractile model based on catch muscle proteins of the Gray's mussel (Crenomytilus grayanus). The objective of this model was to test the current hypotheses about the catch state, a unique phenomenon observed in the adductor muscle of bivalve molluscs. This state allows the muscle to maintain the force developed by contraction for a long time with minimum energy expenditure. Our study has resolved the issue of constructing a catch muscle model capable of controlled contraction and relaxation in an ATP-resistant manner. As a result, we have gained insight into the functional role of myorod, a unique protein of the catch muscle, and have tested the modified myorod-links hypothesis of the catch state. Myorod was shown capable of forming strong and functional cross-links between actin and synthetic thick filaments. This finding removes the last shortcoming of the myorod-links hypothesis of the catch state. There is now no doubt that myorod plays the main role of the catch-link we have been seeking all this time., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Adenosine diphosphate (ADP) is a nucleotide that is structurally very similar to ATP but lacks one of the two high-energy bonds due to hydrolysis. In muscle studies, ADP is usually considered exclusively as a product formed during myosin cross-bridge cycling and is not otherwise involved in this process. In our study, we question the widely held view of ADP as a final product formed during muscle contraction. Using biophysical and biochemical methods, we managed to show that ADP can act as a substrate for myosins in at least three types of muscles: smooth and striated adductor muscles of bivalves (Mytilidae and Pectinidae), and also vertebrate skeletal muscles. According to our data, the differences in the effect of ATP and ADP on the optical, biochemical, and structural properties of actomyosins are exclusively quantitative. We explain the previous ideas about ADP as a compound capable of inhibiting the ATPase activity of actomyosin by the ability of ATP and ADP to depolymerize the polymeric myosin when the concentration in the medium reaches more than 0.3 mM., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Inc. All rights reserved.)