Proteines denature when heated that's what happening when you see the coulour change of .
means that the form of the protein changes but it is not destroyed and looses nothing of it' nutritional values.
Exessive heating (when you burn the meat) is not good because the fat can transform in to polycyclic aromatics (PCA) these are carcenogenic (think of the fat falling in the bbq fire, that becomes PCA, evaporates and sticks on your meat)
Normally heating your meat normally should not have a (major) impact on protein quality. It will actually help your body to digest the protein fraction more easily and hence creates a better post-meal feeling instead of eating the meat raw .