A protein isoform is a version of a protein having only small differences to another isoform of the same protein, and the slight diversity within their amino acid sequences gives rise to different structural and functional properties. This diversity is generated based on gene regulation through two main mechanisms. The first is a qualitative mechanism where the isoforms can be derived from the same gene
through alternative splicing or from different genes of the same family, and the replacement of isoforms generates diversity among the muscle fibers. The second is a quantitative mechanism in which many genes can be up- or downregulated independently of each other on the basis of factors. Therefore, the proportion between the products of these genes is modified, and new functional or structural features
appear.
In particular, myosin is the most abundant muscle protein, and myosin isoforms determine the histochemical ATPase reaction; hence, fiber types are often indicated by the name of the myosin isoform that is expressed. Therefore, myosin isoforms are generally considered the molecular markers of the muscle fiber type. During muscle contraction, the myosin isoforms work together with other myofibrillar proteins, especially troponin and tropomyosin, which have different isoforms. Based on this, the isoform
compositions of the myofibrillar proteins influence thecontractile and biochemical properties of muscle ,
along with postmortem rigor development, and consequently meat quality, particularly in pork , cattle , rabbit , and sheep.