The term 'KIND OF ACID IN PROTEINS' can be answered by the word 'AMINO', which is a five letter word. Amino acids are organic compounds that are the building blocks of proteins. They are essential for life and are found in all living organisms. Amino acids contain both an amino group and a carboxyl group, making them amphoteric molecules. Amino acids are classified as either acidic or basic depending on the pH of the solution they are in.