This exercise is designed to introduce you to the study of macromolecules. Proteins, DNA, RNA, and polysaccharides such as starch, glycogen, and cellulose are all macromolecules. Macromolecules are formed by connecting many smaller molecules together. The individual components of a macromolecule are referred to as monomers. Proteins are composed of monomers called amino acids. All amino acids have a carboxyl group, an amino group, and a central or alpha carbon. The central carbon of each amino acid contains a side chain that is often referred to as an R group. Amino acids form polymers when the carboxyl group and amino group of two amino acids form a peptide bond as shown in Figure 1. Water and a dipeptide are formed in the reaction. More amino acids can be added to the carboxyl group of this dipeptide until a polypeptide is formed.
There are 20 different amino acids that are found in proteins, and each one has a different R-group. These side chains are very important because they impart each amino acid with different characteristics. Amino acids can be characterized as polar, nonpolar, or charged. Charged amino acids are further characterized as acid or basic. Uncharged amino acids can be considered neutral. Aspartic acid is acidic, lysine is basic, and alanine is neutral. The sequence of amino acids that make up a polypeptide are referred to as the primary structure of the protein. The primary structure determines how the protein will fold, which will determine its function. The shapes within a polypeptide are referred to as the secondary structure. The three dimensional structure of an entire polypeptide is referred to as its tertiary structure.
In this exercise you will use the Bradford assay to determine the protein content of two samples. The Bradford assays is an extremely sensitive assay for protein. The Bradford reagent contains a dye called Coomassie G-250 that can interact with the R-group of specific amino acids. One of your samples is milk. The dominant protein in milk is called casein, which is composed of 224 amino acids. Thirteen of these amino acids react with the dye in the Bradford reagent. These amino acids include one tryptophan, four arginines, four tyrosines, and four histidines.
In this experiment, you will
- Create a standard protein curve using the Bradford assay.
- Determine the protein concentration of milk and a high protein drink.
- Determine if the Bradford assay can detect both proteins and amino acids.
- Determine if the Biuret assay can detect both proteins and amino acids.