Proteins

Areas to be covered General Overview of Proteins Structure of a General Amino Acid Replacing the “R” group of a General Amino Acid with specific Chemical Groups – Giving amino Acids their unique Properties. The joining of amino acids to form: dipeptides, tripeptides and polypeptides by a condensation reaction and identifying the Peptide Bond. The Primary structure of a protein and the consequences of altering the type and sequence of amino acids.

General Overview of Proteins Proteins are macromolecules that have very high molecular masses - typically between several thousand and several million. Protein Molecular Mass Number of amino acids Lyzozyme 12 640 129 Myoglobin 16 890 153 Haemoglobin 64 500 574 Tobacco Mosaic Virus 400 000 000 336 500

Where are proteins found? Well, absolutely everywhere!!! HAIR SKIN THEY FORM A COMPONENT OF BONE NAILS BLOOD THEY ARE FOUND INSIDE CELLS – AND EVEN FORM PART OF THE PLAMSA MEMBRANE What do proteins do? Well, absolutely everything!!! Proteins can: Form a structural role e.g. Bone, cartilage. Transport substances e.g. haemoglobin transports oxygen. Store substances e.g. myoglobin stores oxygen in muscles. From a contractile role e.g myosin and actin in muscle Form enzymes that speed up chemical reactions e.g digestive enzymes Form hormones that perform numerous function e.g. insulin lowers blood glucose levels.

These building blocks are called AMINO ACIDS Form toxins e.g. snake venom Provide a protective function as in antibodies Proteins are a highly diverse group of biomolecules in both their structure and their function. What's amazing is that this incredible diversity is generated from just 20 different types of building blocks that combine and join together to form a protein. These building blocks are called AMINO ACIDS

The General Amino Acid

Replacing the “R” Group Each of the twenty amino acids differ in the nature of the “R” group. The simplest amino acid is called Glycine and has a single hydrogen atom as its “R” group. While alanine has a CH3 (methyl) group as its “R” group. Another amino acid called cysteine has the following group as its “R” group This groups gives cysteine the ability to form a bond called a disulphide bond – this is very important in maintaining the structure of a protein.

Joining amino acids together Amino acids join together by the amino group of one amino acid reacting with the carboxylic acid group of another amino acids. The result of this reaction is the formation of a bond called a peptide bond that holds the amino acids together. A dipeptide is formed Di means two Tri means three Poly means many This reaction is called a condensation reaction as water is made and released.

The Structure of Proteins The Primary Structure The types of amino acids contained in the polypeptide chain and the sequence in which they are joined is called the primary structure.

There is an enormous number of different possible primary structures. Even a change in one amino acid in a chain made up of thousands may completely alter the properties of the protein. Sickle cell anaemia is a condition caused by the change in one amino acid in the the primary structure of the haemoglobin protein. In sickle cell anaemia the amino acid glutamic acid has been replaced by the amino acid valine. This causes red blood cell to look like this:

These are called the α helix and the β pleated sheet. The secondary structure. The polypeptide chain can fold to form two different secondary structures. These are called the α helix and the β pleated sheet. Below is a detailed diagram of the α helix:

You should be able to see the cork screw like structure with hydrogen bonds (dotted lines) being formed between the CO and NH groups. These hydrogen bonds stabilize the helix and make it a strong and rigid structure. Also note that the hydrogen bonds link one turn of the helix with another. Also note that the amino acid side chains (R groups) stick out from the helix.

Below is a detailed diagram of the β pleated sheet: Again hydrogen bonds of formed between the CO and the NH groups, but the R groups alternate in either the up or down position - try to look at the diagram to figure this out, it can be hard to see.

The tertiary structure Here the secondary structure folds to give a complex three dimensional shape: Beta pleated sheet Alpha helix Disulphide bond

http://www.stolaf.edu/people/giannini/flashanimat/proteins/protein%20structure.swf

The quaternary structure. Some proteins have this final structure. Proteins that have several subunits like haemoglobin and myoglobin have the quaternary structure. On the next page is a diagram summarising the structures of proteins.