Amino, for one, is an organic compound that is a key component of proteins. It is made up of an amino group and a carboxyl group, and is an essential building block of life. Amino acids are the building blocks of proteins, and they are also important for many other biological processes. Amino acids are classified as either acidic or basic, depending on the pH of the solution in which they are dissolved. The four-letter word for this is 'ACID'.