Question 2a and b please SIOL350 CELL BIOLOGY PROBLEM SET 2 (50pts) Spring 2018

Question

Question 2a and b please SIOL350 CELL BIOLOGY PROBLEM SET 2 (50pts) Spring 2018 Name SFSU ID 2. Short response (max 2 sentences each IN YOUR OWN WORDS) A. (2pts) Phalloidin is an actin stabilizer, and cytochalasin is an actin depolymerizer, yet both drugs lead to loss of actin-based motility. How can these two drugs with opposite molecular effects lead to the same phenotype? B. (2pts) Please describe the cycle of myosin conformations and actin-binding states that occur in the sarcomere during muscle contraction (i.e. the cross-bridge cycle)

Explanation / Answer

Answer 2a:
Cytochalasin extracted from 3 fungal sources, a Phoma sp. (Rothweiler & Tamm, 1966), Helminthosporium dematioideum (Aldridge, Armstrong, Speake & Turner, 1967), Zygosporium mansonii (Hayakawa, Matsushima, Kimura, Minato, & Katagiri, 1968) (genetics and molecular biology of Entomopathogenic fungi, Advances in Genetics 2016). Phalloidin is also a fungal toxins that is isolated from alkaloid Amanita phallpoides (Cytoskeleton, Medical cell biology 2008).
Cytochalasins inhibits polymerization by binding to the growing end of F actin, without affecting depolymerization whereas phalloidin binds tightly to all the sides of actin filaments thereby reducing the concentration of actin monomer at the growing end of the filament, therefore does not allow depolymerization.
Both the chemicals affect polymerization through different mechanism and it would be more correct to say that both drugs with opposite molecular effects are producing same phenotype.

2b: Sarcoplasm contains fine thread like contractile structure called myofibrils. Actin thin filaments and myosin thick filaments are well organized in each myofibril, in linear fashion called as sarcomeres which is the basic functional units of myofibril. Sarcomere shortening is caused by the myosin filaments sliding past the actin thin filaments, with no change in the length of either type of filament. The thick and thin filaments of sarcomeres form a pattern of alternating light and dark bands. The light bands, called I bands (isotropic) contain only thin filaments. The dark band of sarcomeres called A band (anisotropic). A band contain both thin and thick filaments. A narrow H zone in the center of each A band contains thick but not thin filaments. Supporting proteins that hold the thick filaments together at the center of the H zone form the M line, because it is at the middle of sarcomere.
When muscle contracts, I band shortens. The length of each sarcomere of myofibril decreases when the muscle contracts. However, thick and thin filaments do not shorten during muscle contraction.
Myosin head without ATP locks tightly on to an actin filament. Binding of ATP to myosin head caused a slight change in the conformation of the actin binding site. This reduces the affinity of the head for actin and allows it to move along the filament. Hydrolysis of ATP causes the head to be displaced along the filament by the distance of about 5nm. However, product of ATP hydrolysis, ADP and P remain tightly bound to the myosin head. Release of P causes binding of the myosin head to a new site on the actin filament. This release triggers the power stroke. During which the head loses its bound ADP and returns to the start of a new cycle.
Muscle contraction basically triggered by calcium and two other actin bound proteins like tropomyosin and troponin. Tropomyosin is a fibrous molecule that consists of two chains, alpha and beta that attach to F actin in the groove between its filaments. Troponin, complex of 3 polypeptides, troponins T, I and C. when the concentration of calcium is low, tropomyosin and troponins complex blocks the interaction with actin and myosin therefore muscle does not contract whereas at high concentration, calcium binds to troponin undergo some conformational changes leading to contraction.

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