Proteins

• Steroids—these are completely different from triglycerides and phospholipids.

They involve four carbon rings that are linked together. Figure 4 shows what cholesterol, a common steroid lipid, looks like:

The hormones estrogen and testosterone are also steroids. What these have in common is that they are made with the same four carbon rings but have different side chains. Cholesterol, in particular, is part of the cell membrane of many animal cells.

PROTEINS

Proteins are chains of amino acids, which involves a wide variety of different “nitrogenous” molecules, which contain nitrogen. There are peptides, which are short chains of amino acids, also referred to as oligopeptide chains. There are also polypeptides, which are also just called “proteins”, having very long chains. There are 22 amino acids on earth with only 20 actually encoded for by the genetic code. The remaining two, selenocysteine and pyrrolysine are incorporated into proteins by synthetic mechanisms. Some are considered essential to humans because they must be taken in through food and are not encoded by the human genetic code. There are

different amino acids essential to different organisms. Amino acids are specific molecules characterized by having a side chain, an amine group (NH2), and a carboxyl group (COOH). Figure 5 shows the 22 amino acids on earth:

As you can see, the side chain, referred to by the letter “R” is simplest in glycine, in which R is just a hydrogen atom. Some R side chains, such as cysteine, have sulfur in them, while others, such as phenylalanine, will have a carbon ring.

Proteins, as mentioned, are chains of amino acids. They have four different levels of organization as they are very complex molecules. There is a primary structure, which involves just the order of the amino acids. There are so many different combinations of amino acids, making even the primary structure complex and unique. Then there is the secondary structure. This is the three-dimensional shape of the protein. Some amino acids are attracted to one another, linked by hydrogen bonds (a mild type of bonding between molecules, less strong than covalent bonding) giving a 3D shape to the peptide. The tertiary structure is also a 3D organization of a protein made from the fact that some R side chains are hydrophobic, staying deep inside the protein, while others are hydrophilic, staying on the outside of the protein structure. There are also disulfide bonds between two cystine amino acids that twist the protein even further. The quaternary structure of a protein is that which comes from the combination of two