Proteins are broken down through digestion that begins in the stomach. Proteins are broken down by enzymes known as proteases into smaller polypeptides to provide amino acids for the organism, including the essential amino acids that the organism cannot biosynthesize itself. Aside from their role in protein synthesis, amino acids are also important nutritional sources of nitrogen.Proteins, like carbohydrates, contain 4 kilocalories per gram as opposed to lipids which contain 9 kilocalories and alcohols which contain 7 kilocalories. Proteins can be converted into carbohydrates through a process called gluconeogenesis.