There are 20+ amino acids, each differing only in the composition of the R groups. An R group could be a sulfydrl, another methyl, a string a methyls, rings of carbons, and several other organic groups. Proteins can be either acidic or basic, hydrophilic or hydrophobic. The following table shows 20 amino acids that common in proteins.

Figures from Timothy Paustian, University of Wisconsin-Madison ©2000

Proteins can be described as having several layers of structure. At the lowest level, the primary structure of proteins are nothing more that the amino acids which compose the protein, and how those proteins are bonded to each other. The bonds between proteins are called peptide bonds, and they can have either single bonds, double bonds, triple bonds, or more holding the amino acids into a protein molecule.

At the next level, the secondary structure of proteins, proteins show a definite geometric pattern. One pattern that the protein can take is a helical structure, similar to a spiral staircase. Hair has such a secondary structure. When examined closely, you can see the turns in the proteins of hair molecules. A second geometric pattern is the pleated sheet, where several polypeptide chains go in several different directions. I think of a sheet of paper, or a length of fabric. When viewed closely, silk fibronin, the silk protein, forms such a shape. Skin, although made of more than just proteins, provides another example of a protein with a sheet structure. The following figure shows the pleated sheet secondary structure of silk.

Silk Fibroin

Next, we find a tertiary structure to proteins. Here, we find the three-dimensional structure of the globular proteins, where disulfide bridges puts kinks and bends in the secondary structure. Again thinking about hair, some people have straight hair, some have wavy hair, and some have curly hair. The kinks and bends in the secondary structure causes the curls in hair. Curly hair has more kinks and bends that wavy hair, and straight hair has very few, if any bends.

Psoriasin

At the last, we see the quaternary structure of proteins. This the the form taken by complex proteins formed from two or more smaller, polypeptide chains. The polypeptide chains form pieces of a jigsaw puzzle, that when put together form a single protein. Hemoglobin provides a good example, being made from four polypeptide chains.

Hemoglobin