Clinical Biochemistry Lecture 1

Study Notes

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

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

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

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 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

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