And to transform this molecule into a fully functional enzyme that can basically catalyze this particular reaction, what must happen is calcium ions must bind to these red structures. These red structures are the delta structures and the delta structures are actually calmodulent proteins. Remember that calmodulen is a protein that consents calcium ions. It can bind calcium ions. So in skeletal muscle tissue, when we contract the calcium ions stored in the sarcoplasm reticulum is basically released into the cytoplasm. And once inside the cytoplasm, the calcium ions can basically bind onto these rep structures and once the calcium ions bind onto the rep structures, that transforms this partially active enzyme into the fully active phosphorlase kinase.