Proteins

PROTEINS

Proteins are amino acid in a chain, in which small or large amino acids are bonded together to make different sizes of peptides. Short peptides are called oligopeptides, while long peptides are called polypeptides or proteins. Proteins are largely structural aspects of the cells, but some an be hormones, receptor molecules, transport molecules, and enzymes.

Amino acids are organic molecules with two specific groups: a carboxyl COOH group and an amino group or NH2 group. These are attached to what’s called the alpha carbon. Added also to this carbon atom is a side chain, which differs according to the amino acid. The simplest side chain is just hydrogen molecules attached to the alpha carbon. Figure 33 shows the different amino acids found in nature:

Figure 33.

Amino acids attach to one another through a peptide bond. Other bonds that help to shape the protein are disulfide bonds, ionic bonds, and hydrogen bonds. There are also polar and nonpolar aspects to each peptide chain that will attract like amino acids to one another. This is why proteins have three-dimensional shapes instead of linear shapes.

The formation of a peptide bond is one that is a dehydration synthetic reaction. This means that water is made to form the bond. Dipeptides and tripeptides have two and three amino acids attached to one another, respectively. Oligopeptides have a small number of amino acids—in the range of 20 amino acids. Polypeptides have about fifty amino acids. Proteins are very long and have the ability to be a subunit that combines with other protein subunits. Figure 34 shows what a peptide bond formation looks like:

Figure 34.

Proteins are categorized by specific aspects of their structure. The primary structure of a protein is the arrangement of their amino acids in the sequence. Because of interaction and bonding strategies in different proteins, they will have a secondary structure. This happens because of hydrogen bonds in the amino and carboxyl groups. Some will become helical and some will form sheets. The tertiary structure is what happens between the side chains. Disulfide bridges will occur between certain amino acids in some cases. In other cases, there will be ionic or hydrogen bonding. This gives the 3D shape of the protein. The quaternary structure is the interactions that happen between two different peptides.

Conjugated proteins are those that have some type of non-protein component. If, for example, the peptide is attached to a carbohydrate, it is referred to as a glycoprotein. If the peptide is attached to a lipid, it is called a lipoprotein. These are particularly