1.4 - Proteins Flashcards
How to test for proteins in a sample
Biuret test confirms presence of peptide bond.
Add equal volume of sodium hydroxide to sample at room temperature.
Add drops of dilute copper (II) sulfate solution. Swirl to mix.
= this is to make biuret reagent.
Positive result = colour changes from blue to purple.
Negative result = solution remains blue.
What is the general structure of an amino acid?
- COOH (carboxyl/carboxylic acid group)
- R variable side group consists of carbon chain and may include other functional groups such as benzene ring or -OH.
- NH2 (amine / amino group)
How many amino acids are there and how do they differ from one another?
20 amino acids;
Differ only by side ‘R’ group,
How do dipeptides and polypeptides form?
Condensation reaction forms peptide bond (-CONH-) and eliminates a molecule of water.
Dipeptide - 2 amino acids.
Polypeptide - 3 or more amino acids.
How many levels of protein structure are there?
4 -
Primary, Secondary, Tertiary and Quarternary.
Define the “primary structure” of a protein.
- Sequence, number and type of amino acids in the polypeptide.
- Determined by the sequence of codons on mRNA.
Define secondary structure of proteins
Hydrogen bonds form between Oδ- (slightly negative) attached to -C=O & Hδ+ (slightly positive) attached to -NH.
Describe type of secondary protein structure - alpha helix
- All N-H bonds on the same side of protein chain
- Spiral Shape
- H-Bond parallel to helical axis.
Describe the secondary protein structure; B-pleated sheet
N-H and C=O groups alternate from one side to the other.
Describe ‘tertiary structure’ of a protein. Name the bonds present.
- 3d structure formed by further folding of polypeptide.
Bonds include:
- Disulfide bridges
- Ionic bonds
- Hydrogen bonds
Describe each bond in the tertiary structure of proteins
Disulfide bridges - strong covalent S-S bonds between molecules of the amino acid cysteine.
Ionic bonds - relatively strong bonds between charged R groups (pH changes cause these bonds to break.)
Hydrogen bonds - numerous and easily broken.
Describe “quaternary structure” of a protein
Functional proteins may consist of more than one polypeptide.
Precise 3D structure held together by the same types of bond as tertiary structure.
May involve addition of prosthetic groups - e.g. metal ions, or phosphate group.
Describe the structure and function of globular proteins.
Spherical and compact.
Hydrophilic R-groups face outwards & hydrophobic r-groups face inwards -> usually water soluble.
Involved in metabolic processes such as enzymes and haemoglobin.
Describe the structure and function of fibrous proteins
Can form long chains or fibres
Insoluble in water
Useful for structure and support (e.g. collagen in skin)
Outline how chromatography could be used to identify the amino acids in a mixture
- Use capillary tube to spot mixture onto pencil origin line and place chromatography paper in solvent
- Allow solvent to run until it almost touches other end of paper. Amino acids move different distances based on relative attraction to paper and solubility in solvent.
- Use revealing agent or UV light to see spots.
4, Calculate Rf values and match to database.
