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Structure of Proteins:
The structure of an amino acid:
Proteins are large and complex molecules. If a water molecules was the size of a brick (Mr = 18) then
proteins would be an entire building. Amino acids are the basic monomer unit which combine to make a
polymer known as a polypeptide. Polypeptides can be combined to form proteins. All amino acids
contains an amino group, (NH2) a carboxylic acid group, which gives the amino acid the `acid' part of its
name, (COOH) a hydrogen atom and an R group, and with each amino acid there is a different R group.
Both of these groups are attached to a central carbon atom, known as the carbon. The `backbone' of
an atom is made up from the three atoms CCN.
Relative molecular masses of several proteins:
Protein Relative Molecular Mass
Glycogen phosphorylase 370,000
Glutamine synthetase 592,000
How amino acids form proteins:
Amino acids join together in long chains to form proteins by means of peptide bonds. Amino acid
monomers combine together to form a dipeptide. Amino acids will join up through the process of a
condensation reaction, in which the removal of a water molecule occurs. The water is made by
combining the OH from the carboxyl group of one amino acid with the H from another amino group.
The two amino acids then become linked by a peptide bond and are then known as a dipeptide.
Scientists will then look at the structure of peptide bonds at primary, secondary, tertiary and quaternary
levels, to further study them.
The Primary Structure The sequence of amino acids in polypeptide chains:
Through a series of condensation reactions, many amino acid monomers will join together in a process
known as polymerisation this results in a chain of many hundreds of amino acids known as a
The sequence of the amino acids in a polypeptide chain form the primary structure of any protein, and it
is these primary structures that determines the ultimate shape, and therefore function of the protein. A
change in just one amino acid in the primary sequence can lead to change in shape of the protein and
inhibit its function. Much like an enzyme substrate, a proteins shape is very specific to its function,
change the shape and it will function less well, if at all.
The code for the primary structure is contained in the gene or genes that code for that protein, this code
determines the precise order in which the amino acids are assembled. This order then dictates the way
they will twist and turn to produce precise 3D shapes that allows the protein to carry out the specific
function. The first level of 3D twisting is described as the secondary structure.
The Secondary Structure The shape in which the polypeptide chain forms as a
result of hydrogen bonding:
The linked amino acids that make up the polypeptide posses both NH and C=O groups on either side
of each peptide bond. The hydrogen in the NH group has an overall positive charge, whilst the C=O
group has a negative charge. The two groups therefore readily for weak bonds, called hydrogen bonds.
This causes the long peptide chain to be twisted into a 3D shape.
Different shapes occur as the chains of amino acids will twist around to achieve the most stable
arrangement of hydrogen bonds. The secondary structure refers to the patterns contained within the
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As the patterns exist in different parts of the chain in different proteins, the result is
that there in an almost infinite variety of molecular structures.
The main types are the helix, a spiral which is the most common type of secondary structure. This
occurs due to the amino acid residues (a residue are the remains of a monomer that has been
incorporated into a polymer) in the spiral twisting on their axis, each residue forming a hydrogen bond
with another residue 4 units along.…read more