The term 'KIND OF ACID FOUND IN PROTEIN' can be solved by the crossword answer 'AMINO' (5 letters). Amino acids are organic compounds that contain both an amine group and a carboxylic acid group. They are the building blocks of proteins, which are essential for life. Amino acids are classified as either acidic or basic, depending on the pH of their side chain. Acidic amino acids have a carboxylic acid group, while basic amino acids have an amine group. Therefore, the kind of acid found in proteins is an amino acid.