Contractile Proteins Definition

Proteins that aid in the contraction of muscles. Muscle proteins, as well as those present in many other cells and tissues, are among them. These proteins are involved in localised contractile events in the cytoplasm, motile activity, and cell aggregation phenomena in the latter.

Myosin, the major part of thick myofilaments, & actin, the main component of thin myofilaments, are contractile proteins. The elements of myosin that call important regulatory proteins. Sarcolemma has a protein component as well.

The reactive sites of cyclical association as well as disassociation between actin and myosin during contraction are lateral extensions, or cross-bridges, of the thick myofilaments. Variations in the angle of cross-bridge connections are thought to cause the force that causes the filaments to slide.

Composition and Properties of Actin And Myosin

The composition & properties of myosin must be considered in order to comprehend the physiochemical changes that occur at the cross-bridges. Myosin is a structural & enzymatic protein made up of two polypeptide chains (heavy chains) with molecular weights ranging from 16,000 to 25,000 and four polypeptide chains (light chains) with molecular masses ranging from 16,000 to 25,000.

The heavy chains are folded into a globular section at one end and organised into a double helix to generate a long fibrous component. 

The 4 light chains that make up the spherical part are divided into two groups: 

1. two identical DTNB light chains that dissociate from the globular parts using the thiol reagent 5,5′-dithiobis (2-nitrobenzoic acid)
2. two related but distinct species of “alkali light chains” that dissociate at pH 11.

 Myosin’s actin-binding capability and ATPase activity are its two most important biochemical features. The qualities of the alkali light chains are lost when they are separated from the globular areas, but they are not lost when the DTNB light chains are separated. 

Furthermore, the comparative actin-activated ATPase function of various myosin’s appears to be influenced by the alkali light-chain composition.

When myosin is proteolytically digested by trypsin, two fragments are formed:

1. heavy meromyosin, which is made up of the globular component of the molecule and a short segment of the fibrous section, and 
2. light meromyosin, which is made up of the remaining lengthy fibrous portion of the molecule. The cross-bridges are made up of heavy meromyosin fragments, whereas the thick myofilaments are made up of light meromyosin fragments. 

Sub fragment 1 (S1), the globular head area, and sub fragment 2 (S2), the short fibrous part, are the two sub fragments of heavy meromyosin produced by papain digestion. The S1 fragment contains myosin’s actin-binding capacity, ATP-binding capacity, and ATPase activity.

 The S2 fragment is the flexible connector between the cross-globular bridge’s head as well as the light meromyosin fragment. As a result, the region of the myosin molecule with the characteristic chemical properties of myosin (S1) is structurally situated on the terminal portion of the cross-bridges, where it interacts with the actin of the thin myofilaments.

Two F-actin strands are arranged in a double helix arrangement in the thin. Each G-actin monomer has a corresponding binding site for the myosin S1 segment, and the F-actin strands are polymers of the globular protein G-actin. Actin activates the ATPase activity of the myosin S1 segment when it binds to it.

The cyclic interactions of myosin and actin (association, changes in angulation of cross-bridge connections, and disassociation) and ATP hydrolysis are interdependent and complex in contraction. Actin and myosin are joined at the cross-bridges in the non contracting state, as well as the angle of attachment in between cross-bridge heads as well as the actin filaments is 45°. 

With addition of ATP, ATP is attached to the globular head, and actin and myosin dissociate quickly. The globular head shifts to a new place after the ATP is cleaved to produce a myosin-products complex, allowing the angle of adhesion to become 90° whenever the myosin-products complex recombines with actin filament.

The regulatory proteins troponin & tropomyosin regulate this recombination phase between the myosin-products complex and actin in response to calcium ion concentrations. The contraction force is generated by moving the cross-bridge head to a 45° attachment angle, and the cycle is finished when the hydrolytic products of ATP are detached from the head. The cycle can be restarted by generating ATP by rephosphorylation. Every cycle (stroke) reduces a sarcomere by 12 nm, according to measurements.

Activation of Contractile Protein Encoding Genes

In summary, mammalian cardiac myocytes become postmitotic shortly after birth, and myocardial development is hypertrophic in response to increased strain. Increases in cell size & sarcomeric content characterise hypertrophic cardiac myocytes.

Changes in gene expression, particularly increases in interpretation of constitutively active contractile protein producing genes and reactivation of the embryonic gene program, are at the heart of cardiac myocyte hypertrophy at the molecular level. 

Adrenergic agonists, vasoconstrictors, growth factors, and cytokines all have a role in cardiac hypertrophy in response to increased workload. These autocrine or paracrine systems appear to be involved in mediating hypertrophic reactions in cardiac myocytes, according to evidence from both in vitro and in vivo investigations.

Despite the differences in these ligands’ and receptors’ characteristics, their cellular signalling pathways seem to all converge at MAPK, implying that MAPK is important in hypertrophic responses. 

The fact that introducing a constitutively active MAPK into newborn myocytes causes reexpression of embryonic muscle genes further supports MAPK’s function in hypertrophic responses. The relationship between MAPK activation & changes in gene expression has yet to be established.

Conclusion

Proteins make up the foundation of tissue structure. The most major element of striated skeletal muscle is the myofibers. Their classification is based on muscle tissue’s histological structure. Muscle proteins might be contractile, regulatory, sarcoplasmic, or extracellular. Actin & myosin are most important contractile proteins.

Troponin, tropomyosin, M-protein, β –, gamma-actin, and C-protein are all important regulatory proteins. Sarcolemma has a protein component as well. The sarcoplasm contains proteins including myoglobin, myogen, myo albumin, and x-globulin, while the muscle contains proteins like elastin, collagen, and reticulin. 

Myofibrillar proteins from the Z-disc, as well as minor amounts of other proteins, make up the remaining proteins.