Proteins Flashcards
What are proteins
Contain the relents C,H,O,N and sometimes Sulphur
They are polymers of the monomer amino acids (20 different amino acids)
Structure of an amino acid
Amine Group - NH2
Carboxylic acid group - COOH
R group - The rest (anything from a hydrogen atom to a more complex ring structure)
General Structure :
R
I
H2N-C-COOH
I
H
R Groups
R groups have different properties:
- Acidic (-ve charge)
- Basic (+ve charge)
- Polar (attracted to H2O)
- Non polar (hydrophobic/repel water)
Charged R groups
+ve and -ve side chains can be attracted to each other
Polar R groups
Polar side attract water
Hydrophobic R Groups
Hydrophobic attract eachother and repel water
How do amino acids join together?
Amino Acid + Amino Acid —> Dipeptide + H2O
-CONH- = peptide bond
Peptide Bond Formation
Between the OH on the carboxylate group and the H on the amine group
Condensation reaction
H2O as a product alongside dipeptide
Amine group on one amino acid and Carboxyl group on another amino acid react to form a peptide bond
What is primary structure
Refers to the sequence of amino acids in the polypeptide chain and the precise order of them
The sequence determines the specific shape and function of the protein
What is secondary structure?
Folding or coiling of the polypeptide chain because of hydrogen bonding between the amino acids
Twisted to make an alpha helix
Folded/Bent to make a beta pleated sheet
The structures are held together in place by hydrogen bonds
Where do the hydrogen bonds form?
Between th C=O groups of the carboxyl (COOH) group of one amino acid and the H in the amine (NH2) group of another amino acid
What is tertiary structure?
Further twisting and folding of the secondary structure
Caused by:
Hydrogen bonds (weak)
Ionic bonds
Disuphide bridges/bonds (S-S) - strong
so some amino acids must contain sulphur
Tertiary structures are specific and form a unique 3D shape
Tertiary structures : Hydrogen Bonds
Can form between the R groups of polar amino acids
Form between O and OH
Tertiary Structure : Ionic Bonds
Form between a positive and negatively charged side chain of basic and acidic amino acids
Basically, form between oppositely charged R groups
Tertiary Structure : Disulphide bonds
Form as a result of an oxidation reaction between sulphur atoms (e.g 2 cysteines)
Only sometimes occur as there must be a sulphur in the R groups for this bind to occur
Tertiary structure : Hydrophobic regions
Repel water
Hydrophobic regions attract eachother
On the inside of the proteins
What is Quarternary structure?
A protein made up of more than 1 polypeptide chain, each with a tertiary structure combined to form a larger protein complex
Held together by hydrogen bonds,ionic bonds and disulphide bonds/bridges
Can have non protein groups called prosthetic groups
Summary of Primary, Secondary, Tertiary and Quaternary structure
Primary :
Amino Acur sequence/order
Peptide Bonds
Sequence determined specific shape and function of the protein
Secondary:
Hydrogen Bonds
Alpha Helix
Beta pleated sheet
Tertiary:
Further folding and twisting
Hydrogen, Ionic and Disulphide bonds
Quarternary:
More than 1 polypeptide chain
Denaturation of Proteins
- An alteration of tertiary structure and loss of 3D shape which is often irreversible so the protein cannot function
Causes of protein denaturation
High temp=hydrogen bonds break
Changes in pH = Ionic bonds disrupted
Heavy metals = Bind to sites on protein
Detergents = Disrupt hydrophobic interaction
Reducing agents = Break disulphide bonds/bridges
How can a small change in primary structure lead to a non functional protein?
Change in sequence of amino acids (primary structure)
Hydrogen bonds form in diffent places —> alters secondary structure
Further hydrogen, ionic and disulphide bonds form in different places —> different tertiary structure
What if the protein is an enzyme
Shape of active site is altered —> no longer complementary to substrate —> cannot catalyse reaction
Biuret test for proteins
1) Add biuret reagent
2) A purple or lilac colour indicates protein is present
3) If the solution remains blue (copper sulfate colour) then no protein is present
Amino Acid chromatography
Chromatography used to separate mixtures based upon :
Solubility in solvent
Absorbency by chromatography paper
