WBR0746

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::A researcher is studying the mechanisms of muscle contraction. As part of his experiment, he develops a new protein that has high affinity to a special component in the muscle complex. The newly developed molecule is capable of binding to a protein that normally covers myosin-binding sites on actin. The protein developed by the researcher most likely binds to which component of the muscle complex?]]
Answer A	AnswerA::Troponin
Answer A Explanation	AnswerAExp::Troponin is required to expose the myosin-binding sites on actin
Answer B	AnswerB::Tropomyosin
Answer B Explanation	AnswerBExp::Tropomyosin normally covers the myosin-binding sites on actin.
Answer C	AnswerC::Adenosine triphosphate
Answer C Explanation	AnswerCExp::ATP is the form of energy required by the muscle to perform its cross-bridge cycling.
Answer D	AnswerD::Mitochondrion
Answer D Explanation	AnswerDExp::Mitochondrion is a cellular organelle needed to produce ATP. It is abundant in muscles, but has no direct role in the contraction process.
Answer E	AnswerE::Calcium
Answer E Explanation	AnswerEExp::Ca is needed by troponin as a co-factor to expose the myosin-binding sites on actin.
Right Answer	RightAnswer::B
Explanation	[[Explanation::Contraction of muscle cells requires 2 co-factors: adenosine triphosphate (ATP) and calcium (Ca). While ATP provides the energy necessary for the contraction to occur, Ca is required to block the binding of myosin to actin. Physiologically, troponin is responsible for exposing the actin-myosin binding sites by shifting the position of tropomyosin away from the myosin-binding sites on actin. As binding sites are uncovered, myosin can bind to actin. The process of "cross-bridge" cycling thus begins. Cycling releases energy from ATP in the form of adenosine diphosphate (ADP) and phosphate. In the absence of ATP, however, this energy-dependent cycling process may not occur. As such, the absence of ATP marks a phenomenon called "rigor mortis", where the muscles are continuously contracted due to the sustained actin-myosin binding. Educational Objective: Tropomyosin plays a role in covering the myosin-binding sites on actin. References: Krans JL. The sliding filament theory of muscle contraction. Nature Education. 2010; 3(9):66]]
Approved	Approved::Yes
Keyword	WBRKeyword::Troponin, WBRKeyword::Tropomyosin, WBRKeyword::Calcium, WBRKeyword::ATP, WBRKeyword::Contraction, WBRKeyword::Muscle, WBRKeyword::Myosin, WBRKeyword::Actin
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Order in Linked Questions	LinkedOrder::