Proteins are relatively large and complex molecules essential for life. They are classified as biomolecules - along with carbohydrates and DNA and RNA, other molecules that are necessary for life.
Proteins are essentially polymers - that is, they are made up of certain building blocks that are simply used over and over along the chain of the molecule, in this case protein. These building blocks, or monomers, in the case of proteins are molecules referred to as amino acids. By breaking down this name, it can be inferred what it is made of - amino referring to the amino group (-NH2), and acid to the carboxylic acid group (-COOH).
The amino is a base, while COOH is an acid. The amino group can protonated to form -NH3+. Meanwhile, the acid group, COOH, can be deprotonated to form the conjugate base -COO-. Amino acids at physiological pH (around 7) will have this form. They are neutral but contain an equal number of positive and negative charge - they are zwitterions.
These two functional groups are connected to a central alpha carbon. There are 20 amino acids that are essential for life. Of these 20, 19 have a chiral alpha carbon. The presence of this chiral center makes amino acids asymmetric, and are non-superimposable with their mirror images. In simple terms, a carbon will be chiral if it is attached to four unique groups. As mentioned, 19 of the essential amino acids are chiral and hence have a central alpha carbon attached to 4 different groups. The first two groups are the two main functional groups making up amino acids. The other two are 1) a hydrogen, and 2) an R group referred to as a side chain. The only amino acid that does not have a chiral carbon is glycine, because the R group is another hydrogen.
So far, it is evident that amino acids are composed of atoms of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). In fact, this makes up most of proteins. The only other atom that may be found in proteins is sulfur (S) and it is present in the side chain of two amino acids, namely cysteine and methionine.
In brief, proteins are made up of monomers called amino acids. These amino acids are molecules composed of atoms of carbon, hydrogen, nitrogen, oxygen, and in some cases, sulfur. Amino acids, the monomers, are connected via peptide bonds, to form the polymer that is the protein.