The+Structure+of+Proteins

Proteins are the the primary products of genes. They are polymers if amino acids, linked into polypeptide chains by peptide bonds The sequence of amino acids that make up the primary structure of a protein is specified by a gene. The primary structure of the protein is the sequence of amino acids in a polypeptide chain.

Learning Objectives:
 * Describe at least 3 different functions of proteins and give examples (eg. Structural, transport)
 * Describe the four different types of protein structure
 * Relate the primary sturucture of a protein to the way it is folded into a three-dimensional shape

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__Collaborative Notes__ "The function of a gene is to code for the production of a protein". This therefore means one gene codes for one polypeptide which codes for specific sequences of amino acids to produce a specific protein. Proteins have many functions, these include proteins:
 * acting as biological cataylsts, **enzymes**. They allow the chemical reaction of life to occur.
 * having a **structural** function. For example in vertebrates, the protein collagen in tendon, ligament and bone, and the protein keratin in hair, feathers, claws, nails and epidermis.
 * being **hormones.** Some hormones are proteins, such as insulin, which controls the the level of glucose (sugar) in the blood.
 * being **antibodies**. They help defend more complex animals against disease.
 * having a **transport** function, but only some proteins. Example is haemoglobin which helps transport oxygen to the blood cells
 * being **contractile**, meaning they are responsible for movement. Only some porteins, such as actin and myosin of muscle and tubulin that makes up spindle fibres in cell division

Proteins are made of one or more chains called **polypeptides**. These polypeptides consist of several thousands of **amino acids**. These are linked together with peptide bonds in a particular order which code for a specific type of protein to be produced. Any mutations which occur in the base sequence of a gene, can possibly change the amino acid produced in the polypeptide (however may not, because of the degeneracy of amino acids), and therefore will change the specific sequence and hence alter the protein produced, or will cause it to be non-functional. There are two main types of proteins, according to the shape of the molecule.
 * **Fibrous proteins -** proteins that have many polypeptide chains which form rope-like bundles, examples are collagen in tendon, ligament, and bone and keratin in the epidermis of the skin, and structures produced from it such as nails, hair and feathers.
 * **Globular proteins -** are when the polypeptide chain is irregularly folded into a ball-shaped molecule, example of this are enzymes. They are major components of cell membranes.

There are four different types of structures of a protein. __**Membrane Proteins**__ Some membrance proteins transport substances in and out of a cell, while others help to detect chemical signals from other cells.They are held by hydrogen bonding between R- groups and water and are located floating in the lipid bilayer.
 * **Primary structure -** are the sequence or order of amino acids along the polypeptide chain. Each kind of protein has a primary structure that differs from that of every other protein. The amino acid sequence determines the properties of the protein.
 * **Secondary Structure** - are in both fibrous and globular proteins. The polypeptide chain is bent or twisted to form a geometrically regular secondary structure. The 2 common kinds are the alpha helix and the beta arrangement or pleated shet.
 * **Tertiary Structure** - are in globular proteins where the polypeptide chain is folded irregularly to form this structure, these vary greatly from one protein to another.
 * **Quaternary Structure** - are in many globular proteins which consist of two or more polypeptide chains held loosely together to form this structure.