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Protein structure and diversity

Table of Contents
  1. Introduction
  2. Structure of proteins
  3. To Go Further

Introduction

It is difficult to describe in a simple sentence the role of proteins. Lets say:

when there is something to do, it is a protein that does it.

Therefore proteins are

Some examples of proteins

The role (or function) of a protein depends on its shape, and chemical formula. Therefore, the structure of proteins will be explained first.

Structure of proteins

Proteins are synthesised primarily during a process called translationask Dr Chromo!. The building blocks of the proteins are amino acidsask Dr Chromo! . Proteins are made of a long chain of amino acids, sometimes modified by the addition of hemeask Dr Chromo!, sugars, or phosphatesask Dr Chromo!.

Primary Structure

This is the sequence of amino acids, which form a chain connected by peptide bonds. The amino acid sequence of a protein determines the higher levels of structure of the molecule. A single change in the primary structure (the amino acid sequence) can have a profound biological change in the overall structure and function (e.g. Please refer to the chapter about genes where examples are given).

Note: If there are some cysteines in the amino-acid sequence, they often react two by two to form disulphide bridges. Disulphide bridges are part of the primary structure.

The primary structure of a protein is its amino acid sequence and the disulphide bridges, i.e. all covalent connections in a protein.

Primary structure of a protein

Secondary Structure

secondary structure

The secondary structure is the way a small part, saptially near in the linear sequence of a protein folds up into:

  • Alpha - Helix: the first structure described by Linus Pauling. It has a rod shape. The peptide is coiled around an imaginary cylinder and stabilized by hydrogen bonds formed between components of the peptide bonds.


  • Beta - pleated sheets: the amino acids adopt the conformation of a , sheet of paper and the structure is stabilized by hydrogen bonds between amino acids in different polypeptide strands. Note that some of the strands are parallel and some are antiparallel.


  • Other parts of the structure are not highly stable, and adopt a random coil formation.




    • Tertiary structure

    The tertiary structure (or conformation) is the way random coils, alpha -helixes and beta -pleated sheets fold in respect to each other, ie it refers to the protein as a whole.

    Amino acids which are very distant in the primary structure might be close in the tertiary one because of the folding of the chain.

    tertiary structure

    Quaternary structure

    Quaternary structure

    The quaternary structure is the arrangement of polypeptide subunits within complex proteins made up of two or more subunits, sometimes associated with non proteic groups.

    This image shows a dimer (two monomers) of a simple protein.

    A good example is the quaternary structure of haemoglobin, made up of two alpha and two beta polypeptide chains.

    When you have loaded the molecule into RasMol, try to type the following commands in the Command window:

    Now the quaternary structure of the protein should be easy to spot.

    Examples of proteins

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