The term 'KIND OF ACID IN PROTEIN' can be solved by the crossword solution 'AMINO' (5 letters). Amino acids are organic compounds that are the building blocks of proteins. They contain an amino group and a carboxyl group, and are essential for all living organisms. Amino acids are classified into two categories: essential and non-essential. Essential amino acids cannot be synthesized by the body and must be obtained from food sources, while non-essential amino acids can be synthesized by the body.