What is the X in X-mas?
troponin tro·po·nin (trō'pə-nĭn, trŏp'ə-)
A calcium-regulated protein in muscle tissue occurring in three subunits with tropomyosin.
One of the proteins that make up the thin filaments of muscle tissue and that regulate muscle contraction and relaxation. Troponin occurs in three forms bound together in a complex. One of the three forms is a receptor of calcium ions that induces structural changes that allow the actin in the thin filaments to interact with myosin, causing contraction.