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BIOL 4510 (34)

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BIOL 4510
Peter Backxx

Regulation of contractile proteins by phosphorylation four contractile proteins are phosphorylated TnI Cprotein TnT and MLC2 Phosphorylation by PKA occurs in Nterminal region of TnI unique to cardiac TnI 2TnI phosphorylation phosphorylated by PKA reduces affinity of Ca binding to 22TnC by enhancing Ca unbinding rates causes rightward shift in forceCa relationship 2 since contractile proteins not Ca controls the maximum speed of force relaxation it is essential for the contractile system to relax more quickly when the sympathetic system is activated remember heart rate can go up 34 times during sympathetic 2stimulation increased speed of Ca cycling in cytosol discussed in next lecture is insufficient for a proper response 2 by phosphorylating TnI the rates of Ca binding and unbinding from contractile proteins is enhanced thereby increasing rates of pressure and force relaxation 2Figure 25 Effects of betaadrenergic stimulation activation of PKA on the forceCa relationshipPKAdependent phosphorylation of MyBPC and MLC2 are also critical for ensuring proper responses of the contractile system to increased demand for cardiac output Both speed up crossbridge cycling rates 1 2Ca cooperativity in contraction of cardiac muscle 2 steadystate forceCa relationship are characterizedquantified by the Hill equation2n2n FforceF CaCaEC max50 22where Ca is the free Ca concentration Fthe maximal force max2that the muscle can generate at high Ca n is the Hill coefficient2 measure of cooperativity ECthe Ca corresponding to half50maximal levels of force05M2 considering that 1 Ca binds per cTnC molecule n is expected to be 1 this is the predicted dependence from onetoone binding theoryn is actually about 56 When a parameter like force depends strongly on the concentration of the a substance biochemists characterize these properties as reflectingA famous example of cooperativity is O binding 2to hemoglobin Haemoglobin has 4 binding sites that can bind 4 O molecules2It turns out that each O that binds hemoglobin increases the affinity of the 2next O binding ie binding gets stronger with each O In fact the 22description of O binding to hemoglobin by AV Hill lead to Hill equation2 High n means the muscle acts like an onoff switch full activation occurs over a 2small range of Ca levels2 Note this does not mean that less Ca binds to TnC In fact for full activation the 2same amount of Ca has to be bound regardless of the Hill coefficient2
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