Clinical Biochemistry Lecture 1

Classification of Muscles

• There are two main types of muscles: striated muscles and smooth muscles
• Striated muscles are further classified into skeletal muscle (attached to bone) and cardiac muscle (found in the heart)
• Smooth muscle cells appear non-striated and are found in the intestine, stomach, and lung

Structure of Skeletal Muscle

• Muscle fibers are multinucleated and surrounded by the sarcolemma (plasma membrane)
• Each muscle fiber is composed of many myofibrils
• Myofibrils are made up of bundles of myofilaments
• Myofilaments consist of thick filaments (myosin) and thin filaments (actin, tropomyosin, and troponin)
• Myofilaments are the actual contractile elements of striated muscle
• Sarcomere represents the smallest functional unit of myofibril

Protein Composition of Muscle Fibers

• Muscle contains two main types of proteins: contractile proteins and regulatory proteins
• Contractile proteins include myosin and actin
• Regulatory proteins include tropomyosin and troponin

Myosin

• Each myosin molecule is composed of 6 polypeptide chains (2 heavy chains and 4 light chains)
• The heavy chains twist around each other to form a double helix (tail of the myosin molecule)
• The amino terminal end of each heavy chain folds into a globular structure (myosin head)
• Myosin exhibits ATPase activity, which hydrolyzes ATP to ADP + Pi and provides free energy for muscle contraction
• Myosin interacts with actin to generate the force that moves the thick and thin filaments past each other

Actin

• Actin is the major constituent of thin filament of muscle fiber
• Actin is a polymer of globular-shaped subunit called G-actin
• G-actin polymerizes into a fibrous or filamentous form called F-actin in the presence of Mg2+ ions
• Each G-actin molecule has a binding site for myosin
• Actin plays a major role in muscle contraction in association with myosin

Regulatory Proteins

Tropomyosin

• Tropomyosin is a constituent of thin filaments of the muscle
• Tropomyosin is made up of two polypeptide chains wrapped spirally around the sides of the F-actin helix
• In the resting state, tropomyosin molecules lie on top of the active sites of the actin strands, preventing attraction between actin and myosin filaments to cause contraction
• Tropomyosin regulates the attachment of actin and myosin during contraction

Troponin

• Troponin is a constituent of thin filaments of the muscle
• Troponin is involved in the contraction process by regulating the attachment of actin and myosin