The liver and kidney play very important roles in the metabolism of nitrogenous compounds. The presence of all eight essential amino acids in a food source define a complete protein, that is a single source of all eight amino acids in the right proportions. Animal protein is complete protein, but is often high in fats, which mean that while a good source of protein, one needs very little meat to fulfil dietary requirements and often one overconsumes both protein and fats. Plant protein on the other hand is incomplete protein, that is missing one or more of the essential amino acids or having them in disproportionate amounts. This means that in order to get all of these amino acids, one has to consume more than one type of plant protein, but there is little fat in plant products and no cholesterol.
Protein can have either structural or functional roles in biological systems.
Among structural protein products are hair, nails and muscle. Functional examples include hormones, antibodies and enzymes. Enzymes are organic catalysts which speed up metabolic reactions. Of particular importance in enzymatic activity is the active site or region of enzyme specificity. If the enzyme structure is changed, the activity is usually lost.