BM: Proteins Flashcards
What are the monomers of proteins?
Amino acids
What are amino acids?
The monomer unit which combine to form polypeptides.
Polypeptides then combine to form proteins.
What is formed when two amino acids join?
A dipeptide
What is formed when more than two amino acids join together?
Polypeptides.
What are proteins made up?
One or more polypeptides.
What is the general structure of an amino acid?
How many amino acids have been discovered?
How many of these are naturally occuring.
About 100 have been identified.
20 of which naturally occur in all living organisms - this provides evidence for evolution.
What is the only difference between the 20 naturally reoccuring amino acids?
The ‘R’ variable group.
Glycine is the only amino acid that doesn’t have carbon in its R group. It consists of just one hydrogen atom.
What bond is formed by the joing of two amino acids?
A peptide bond.
Draw a condensation reaction between two amino acids:
How do amino acids join together?
Through condensation reactions.
A water molecule is released and a peptide bond forms.
How many structural levels do all proteins have?
How many can some larger proteins have?
All have 3 levels - primary, secondary and tertiary.
Some have 4 levels - an additional quaternary structure.
Through what process can many amino acids join together?
Polymerisation.
This results in the formation of a polypeptide.
What forms the primary structure of a protein?
The sequence of amino acids in the polypeptide chain.
What determines the sequencing of amino acids in the primary structure of a protein?
DNA.
What determines the ultimate shape and function of a protein?
The primary structure.
A change in sequencing of amino acids results in bonds forming in different places, and the protein folding differently.
This changes its shape which is very specific to its function.
What causes hydrogen bonds to form in the secondary structure of the protein?
- Linked amino acids contain both -NH and -C=O groups on either side of peptide bond.
- H of -NH group has an overall positive charge.
- O of -C=O group has an overall negative charge.
- Therefore, a weak hydrogen bond is formed between them.
Describe the secondary structure of a protein:
- Hydrogen bonds form between amino acids in chain.
- Therefore, chain becomes a 3D shape by either:
- Coiling into an alpha helix
- Folding into a beta pleated sheet.
Draw a diagram illustrating the difference between an alpha helix and a beta pleated sheet:
What bonds are there within the primary structure of a protein?
Peptide
What bonds are there within the secondary structure of a protein?
Hydrogen
What bonds are there within the tertiary structure of a protein?
- Disulfide - fairly strong and not easily broken
- Hydrogen - numerous but easily broken
- Ionic - formed between carboxyl and amino groups not involved in peptide bond - weaker than disulfide and broken by changes in pH.
Describe the tertiary structure of a protein:
- Polypeptide chain is coiled or folded further.
- More bonds form between different parts of the chain, such as ionic and hydrogen.
- Disulfide bridges form.
- Can result in final 3D structure.
How do disulfide bridges form?
When two molecules of the amino acid cysteine come close together - sulfur atom in one molecule bonds to sulfur atom in another.
What is important in determining how a protein functions?
Its 3D structure - this is distinctive allowing it to recognise, and be recognised by, other molecules.
This is first determined by the sequencing of amino acids.
Describe the quaternary structure of some proteins:
- Two or more polypeptide chains joining together.
- Sometimes, there may also be a non-protein group associated with the molecule.
- Eg, the haem group in haemoglobin.
Give examples of proteins that have a quaternary structure:
- Haemoglobin
- Insulin
- Collagen
Draw a diagram to illustrate the structural levels of a protein:
Name 4 different examples of proteins:
- Enzymes
- Antibodies
- Transport proteins
- Structural proteins
Explain the structure of enzymes:
Usually roughly spherical due to tight folding of polypeptide chains.
Describe the structure of antibodies:
Two light polypeptide chains and two heavy polypeptide chains bonded together, with a variable region so they can be highly specific.
Describe the structure of transport proteins:
Hydrophobic amino acids and hydrophilic amino acids are both present, causing the protein to fold up and form a channel.
Describe the structure of structural proteins:
Long polypeptide chains lying parallel to each other with cross-links between them, making them very strong.
What is the test for proteins?
Biuret test
What does the biurets test detect?
The presence of a peptide bond.
How would you perform the Biuret test?
- Place sample in test tube.
- Add an equal volume of sodium hydroxide solution at room temperature, to make the solution alkaline.
- Add a few drops of very dilute (0.05%) copper (II) sulfate solution.
What are the possible results of the Biuret test?
Positive = solution turns purple
Negative = solution remains blue
The colours are pale, so you will need to look carefully.
