Protein as a source of energy tops out at about 10% though some studies suggest as low as five. Also there is no natural store for protein, just the facilitating of broken down amino acids to body tissues. The only time the body stores it is if you are breaking down muscle tissue on a regular basis, your body will then store it to a small degree still, for future uses. Which i find to be quite interesting.
When in starvation or complete depleting your body will turn to anything other than protein, it really is the last resort. This is why low carb diets work the way they do because your body will keep turning to fat stores and then in desperation protein. The problem comes in that because your body doesn't naturally store it and its a low grade energy source anyway, your body will turn to muscle stores and deplete their first. To much protein as a fuel source can inhibit muscle gains. The proper balance is very important. If you are looking for energy fuel source perfection then it truely is the carb/fat/protein combo that is needed for duration and proper delivery to the ATP.
The only time it gets tricky is if you are trying to manipulate it for optimized fat loss. When you are going beyond energy expenditure alone and into fuel sources the best method is to try the trigger the burning of stored fat cells, the only way to do this in a targeted way is to have close to depleted carbohydrates so that fat energy become the main source. Instead of turning to the touted fat burning zone theory, it is better to do the fast burst of carbohydrate depleting with burst of slowed fat burning targets. Hince why HIIT is really a great fat burning activity while protecting of the muscle because it allows both stores to work off one another. In exercise you should not want to access protein, protein fuel should be utilized most when NOT working out so that the amino acids can do their job of delivery to the best of their ability.
