Proteins are large, complex compounds composed of hundreds to thousands of smaller units called amino acids. There are twelve essential amino acids required by dogs whereas cats require thirteen dietary amino acids. In fact, dogs and cats do not have a “protein” requirement per se but have known daily requirements for each essential amino acid.
The ESSENTIAL amino acids MUST be in the diet, but the source of these amino acids (chicken, beef, soybean) is irrelevant. Thus a product claiming to have a particular source of meat (lamb, for example) as the primary ingredient is not nutritionally better for your pet than another brand with a different protein (poultry for example) if they both contain all the essential amino acids.
Since pets have requirements for specific amino acids, the crude protein number on the pet food label is of little value because essential amino acid concentrations are more important.
Protein quality is determined by the type and number of amino acids. Egg and liver are of the highest protein quality because of their amino acid profiles. Amino acids may become incorporated into body tissues, burned for energy, or, if consumed in excess, will be converted and stored as body fat.