Proteins Flashcards
What are amino acids?
Amino acids are the basic monomer units which combine to make polypeptides.
Polypeptides can be combined to form proteins.
How many amino acids are there?
There are 100 identified.
20 amino acids naturally occur in proteins.
The fact the same 20 amino acids occur in all living organisms provides indirect evidence for evolution.
They only differ by their side group.
What is the structure of an amino acid?
A central carbon atom to which are attached four groups.
Amino group (-NH2): a basic group.
Carboxyl group (-COOH): an acidic group.
Hydrogen atom (-H).
R (side) group: a variety of different chemical groups. Each amino acid has a different R group.
What is the formula for an amino acid?
R
{
H2N ————–C————–COOH
{
H
How are peptide bonds formed?
Amino acids combine to form a dipeptide, in a condensation reaction:
The water is made by combining an OH from the carboxyl group with an H from the amino group of another.
They become linked by a peptide bond between the carbon atom of one and the nitrogen atom of the other.
The peptide bond can be broken by hydrolysis to give its two constituent amino acids.
How are polypeptides formed?
Through a series of condensation reactions, many amino acid monomers can be joined together by polymerisation.
The resulting chain of hundreds of amino acids is a polypeptide.
What is the primary structure of proteins?
The number and sequence of amino acids in a polypeptide chain, determined by DNA.
There are a limitless number of possible combinations of primary protein structure.
Primary structure determines its ultimate shape and function.
A simple protein may consist of a single polypeptide chain, but more commonly, a protein is made up of a number of chains.
What happens if an amino acid changes?
A change in just a single amino acid in the primary sequence can lead to a change in the shape of the protein and may stop it carrying out its function or just less well.
A protein’s shape is very specific to its function.
What is the secondary structure of proteins?
The linked amino acids that make up a polypeptide possess both NH and C=O groups on either side of each bond.
The hydrogen of the NH group has an overall positive charge and the O of the C=O group is negative.
These two groups readily form weak hydrogen bonds.
This causes the long chain to be twisted into a 3-D shape.
Forms an α-helix or a β-pleated sheet.
What is the tertiary structure of proteins?
The α-helices of the secondary protein structure can be twisted and folded even more to give the complex, and often specific, 3-D structure of each protein.
The 3D shape is important in its functioning and makes each protein distinctive and allows it to recognise, and be recognised, by other molecules.
It can then interact with them in a very specific way.
How do bonds hold the tertiary structure together?
Where the bonds occur depends on the primary structure of the protein.
Disulfide bridges - are fairly strong and therefore are not easily broken.
Ionic bonds - are formed between any carboxyl and amino groups that are not involved in forming peptide bonds. They are weaker than disulfide bonds and are easily broken by changes in pH.
Hydrogen bonds - are numerous but easily broken.
What is the quaternary structure of proteins?
Large proteins often form complex molecules containing a number of individual polypeptide chains that are linked in various ways.
May also be non-protein (prosthetic) groups associated with the molecules.
For example the iron-containing haem group in haemoglobin.
What is the test for proteins?
The Biuret test, which detects peptide bonds:
Place a sample of the solution in a test tube and add an equal volume of sodium hydroxide at room temperature.
Add a few drops of very dilute (0.05%) copper sulfate solution and mix gently.
A purple colouration indicates proteins, stays blue if not.
What are the two basic types of protein?
Fibrous proteins, such as collagen, have structural functions.
Globular proteins, such as enzymes and haemoglobin, carry out metabolic functions.
What are fibrous proteins?
They form long chains which run parallel to one another.
These chains are linked by cross-bridges so form very stable molecules.
