Suppose that the amount of troponin complex in fast twitch muscle fibers is abou

Question

Suppose that the amount of troponin complex in fast twitch muscle fibers is about 70 mM. The density of muscle is about 1.06 g/cm3 .

a. From the structure of the thin and thick filaments, estimate the amount of myosin heads. Convert this to nmol per g of tissue.

b. Suppose that the muscle activation is 75 ms and that the turnover of the heads is 10/s. About how many ATP would be hydrolyzed in a single twitch per g tissue? Use a step function for activation: It’s on or off with a 75 ms duration. This is a crude estimate.

c. Assume that the troponin complex binds 2 Ca2+. How much ATP would be split by the SERCA pumps to return activating Ca2+ to the SR per g tissue?

Explanation / Answer

1. The Troponin complexes occur at 38.5 nm intervals, and the thin filaments are about 1000 nm long. That corresponds to about 26 troponin per thin filament for each actin helix. But there are two actin helices per thin filament, so the number is closer to 52 Troponin per thin filament. There are two thin filaments for each thick filament. So, there are total 104 troponnin complexes for each half of the thick filament.

Each half thick has a myosin head every 14.3 nm, and this extends about 680 nm on each half of the thick filament (830- 150 nm bare zone in the middle of the filament). This corresponds to about 48 myosin group per thick filament. Each of these are doublets consisting of two myosin heads. SO, the number of myosin heads on half thick filament is 96.

these calculations establish the ratio between Troponin and mysoin heads, which is close to 1.:

104troponin/ 96 myosin= 1.08 Troponin/ myosin . Thus, if TnC is about 70 um, we would expect about

70 / 1.08= 65 um (micronmeter) myosin

This is 65 nmol of myosin per cm3 of the tissue which is 65 nmol/1.06g= 61.3 nmol g-1

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