General Structure of the amino acid molecule

In this diagram the R represents the variable that makes each amino acid different, the chemicals bonded with carbon in place of the R distinguishes the amino acid.

Condensation Reaction

A condensation reaction is where two molecules combine into a more complex molecule, this results in the elimination of a simple substance, most commonly water. Lots of Polymers are created through a condensation reaction. It is the opposite reaction to hydrolysis which is breaking down large molecules into smaller ones, by adding water.

Functions of Proteins

Most proteins have a specific function to undertake, these can be divided into roughly seven different groups. Transport, Structural, Defence, Transport into and out of cells, Hormones, Enzymes and contractile proteins.

Transport proteins are used to carry vital substances around the body, like haemoglobin carries oxygen around the body. Structural proteins are used to provide connective tissue; it also makes hair and nails. Defence proteins include immunoglobulin that protects the body against foreign pathogens. Transport into and out of cells is also carried out by proteins, called carrier and channel proteins, these regulate movement across the cell membrane. Many hormones also have a protein structure such as glucagon and insulin, both integral for monitoring the body’s blood sugar levels. Almost all enzymes are proteins too, these enzymes are used for breaking down foodstuffs for the body to use. Contractile proteins are used in muscles to allow contraction, muscles are needed to move and actin and myosin allow them to do so.

Primary Structure

Primary structures of proteins are the result of many condensation reactions in which many amino acid “monomers” are joined together in polymerisation. The resulting chain makes a polypeptide. These amino acids joined together to make a polypeptide chain forms the protein’s primary structure.

Secondary Structure

Alpha-Helix

Secondary structures of proteins are divided into one of two three dimensional forms, either alpha-helix or beta-pleated sheet. This is because hydrogen of the –NH group has a positive charge while the O of the –C=O group has a negative charge, so they form hydrogen bonds, causing polypeptide chains to twist.

Beta-pleated Sheet

In the alpha helix the polypeptide chain is coiled into a cylindrical shape. In Beta-pleated sheet the polypeptide chains are linked in a parallel flat sheet.

Secondary Structure Bonds

The bonds that hold the secondary structures in place are hydrogen bonds (as explained above)

Three types of bond that hold a Tertiary Structure

The three types of bond are hydrogen, ionic and disulphate.

Tertiary Structure

Alpha helices and beta pleated sheets can be twisted or moved into different and more complex forms. These are called tertiary structures and are the result of four different bonds:

• Disulphide bridges. Bonds between sulphur atoms and cysteine. These are covalent bonds and therefore very stable.
• Ionic bonds. Between Carboxyl and amino groups not involved with peptide bonds. These are quite weak and can be broken with changes in pH.
• Hydrogen bonds are bonds between H and O. Positive and negative.

Globular Protein

A Globular protein is a protein with four polypeptide chains, two alpha and two beta. They form together in haemoglobin a spherical globe. They are also associated with a prosthetic group, such as iron, which gives the protein special properties.

Fibrous protein

These proteins form long chains, these run parallel to each other. Linked by cross-bridges they are extremely stable molecules. They are often used for providing physical strength, for instance, in tendons or bone. It is strong and stable while being able to be flexible.

Quaternary structure

Large proteins form complex molecules that contain lots of individual polypeptide chains that are linked. There are also prosthetic groups that can be associated with the molecules. Haemoglobin and insulin are both quaternary structures.