Chemistry of Proteins

    Carbohydrates, fats, proteins and nucleic acids are the four major groups of organic molecules found in living organisms.  This lab deals with the important class of organic molecules known as proteins.  They are the main structural and growth components of cells in tissues such as skin, hair, muscle and blood.  Other proteins serve in a regulatory capacity as enzymes or hormones.  Proteins always contain nitrogen in addition to carbon, hydrogen and oxygen.  Phosphorus and sulfur are also found in many proteins.

    The amino acid is the basic structural unit of all proteins.  There are only about 20 different amino acids known to exist in proteins; all of them have a similar basic structure.  The general formula of an amino acid is shown in Fig. 1.

Materials

  • 14 Carbon (C) - tetrahedral electrons - black

  • 4 Nitrogen (N) - tetrahedral electrons - red

  • 9 Oxygen (O) - double electrons - blue

  • 32 Hydrogen (H) - single electron - white

  • 58 electron bond - plastic tube - white

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    Procedure:

    1. Examine the structural formula for an amino acid in Fig. 1.  Construct a model using the molecular parts provided leaving the R portion open.

    2. Note that the amino acid has an amino (NH2) group at one end and an acid (carboxyl) - (COOH) group at the other end.  R stands for radical (an atom or a group of atoms).  The uniqueness of each amino acid is determined by the atoms in the R position.

    3. The structural formulas of four (4) representative amino acids are shown in Fig. 2.  Glycine is the simplest amino acid, with only a single hydrogen atom in the R position.  Complete the model previously constructed.

    Study the structural formulas of Fig. 2.

        a.  What is the molecular formula of glycine?  (Add the correct subscripts)  C   H   O   N

        b.  What is the molecular formula of alanine?  (Add the correct subscripts)  C   H   O   N

        c.  Are the molecular formulas for all amino acids the same?

            What is the end arrangement found in fatty acids as well as amino acids?

            What is it called?

        The models you have constructed represent the three dimensional shapes of molecules.  A protein is formed by the chemical bonding of many amino acid molecules.  Proteins may contain as few as 50 or as many as 5000 or more amino acids.  The chemical combinations of two amino acids is called a dipeptide.  The amino group of one amino acid molecule combines with the acid group of another in what is called a peptide bond.  In this reaction one molecule of water is formed by the removal of a hydrogen atom from the amino group of one amino acid molecule and an OH group from the acid group of the other.  This process is called dehydration synthesis as is shown in Fig. 3.

    1. Combine the two amino acid molecules built by removing the proper -OH group and -H group as needed to form a protein.  Rejoin the -OH and -H ends.

         a.  What chemical substance is formed when the -OH and -H is joined?

    As in fats and carbohydrates, the reverse of this reaction, called hydrolysis, restores a water molecule at the site from which it was originally removed.
     
    Interpretations:
    1. Dehydration means "water loss".  Synthesis means "to put together".  Explain why the chemical process responsible for building a protein molecule is called dehydration synthesis.
    2. What type of molecule is needed to form protein molecules?
    3. How do amino acid molecules differ from fatty acid molecules?
    4. How might a human muscle protein molecule differ from a horse muscle protein molecule?
    5. What purpose is served by the loss of an -H and -OH end from two molecules as they join together during dehydration synthesis?
    6. A protein consisting of four amino acids undergoes hydrolysis.  How many water molecules must be broken down and reattached to amino acid molecules during this process?