Which regulatory proteins control cross-bridge formation and how does calcium affect them?

Which regulatory proteins control cross-bridge formation and how does calcium affect them?

• A. Ca2+ binds directly to actin to expose myosin-binding sites.
• B. Troponin-Tropomyosin complex; Ca2+ binds to troponin C causing tropomyosin to move away from myosin-binding sites on actin, enabling cross-bridge formation.
• C. Myosin head regulation is controlled by ATP hydrolysis and is not influenced by Ca2+.
• D. Nebulin and titin regulate the cross-bridge by calcium-activated phosphate transfer.

Answer: (B)

Ca2+ regulates cross-bridge formation by controlling the troponin-tropomyosin complex on the actin filament. When calcium levels rise, Ca2+ binds to troponin C within this complex, causing a shape change that moves tropomyosin away from the myosin-binding sites on actin. With those sites exposed, myosin heads can bind actin and form cross-bridges, allowing the contractile cycle to proceed. Without calcium, the troponin-tropomyosin complex blocks the binding sites, preventing cross-bridge formation. The other ideas don’t fit because calcium does not bind directly to actin to expose sites, cross-bridge regulation is not driven solely by ATP hydrolysis of myosin, and nebulin/titin are structural proteins rather than calcium-activated regulators of cross-bridge formation.

Ca2+ regulates cross-bridge formation by controlling the troponin-tropomyosin complex on the actin filament. When calcium levels rise, Ca2+ binds to troponin C within this complex, causing a shape change that moves tropomyosin away from the myosin-binding sites on actin. With those sites exposed, myosin heads can bind actin and form cross-bridges, allowing the contractile cycle to proceed. Without calcium, the troponin-tropomyosin complex blocks the binding sites, preventing cross-bridge formation. The other ideas don’t fit because calcium does not bind directly to actin to expose sites, cross-bridge regulation is not driven solely by ATP hydrolysis of myosin, and nebulin/titin are structural proteins rather than calcium-activated regulators of cross-bridge formation.