1.4 Proteins Flashcards
General structure of an amino acid
- COOH carboxyl group
- R variable side group consists of carbon chain and may include other functional groups e.g. benzene ring
- NH2 amine group
What does Biuret test do?
Confirms presence of peptide bond
Describe how to test for proteins in a sample
- Add 2cm3 food sample to test tube
- Add equal volume of sodium hydroxide to sample at room temp
- Add two drops of dilute copper (II) sulfate solution. Mix
- Positive result: colour changes from blue to purple
How many amino acids are there and how do they differ from one another?
- 20
- Differ only by side ‘R’ group
How do dipeptides and polypeptides form?
- Condensation reaction forms peptide bond (-CONH-) and eliminates molecule of water
- Dipeptide: two amino acids
- Polypeptide: 3 or more amino acids
How many levels of protein structure are there?
4
Define ‘primary structure’ of a protein
- Sequence, number and type of amino acids in the polypeptide
- Determined by sequence of codons on mRNA
Define ‘secondary structure’ of a protein
hydrogen bonds form between O delta-negative (slightly negative) attached to -C=O and H delta-positive (slightly positive) attached to -NH
Describe the 2 types of secondary protein structure
Alpha helix
- All N-H bonds on the same side of protein chain
- Spiral shape
- H-bonds parallel to helical axis
Beta pleated sheet
- N-H and C=O groups alternate from one side to the other
Define tertiary structure of a protein.
3D structure formed by further folding of polypeptide
Name the bonds present in the tertiary structure of a protein
- Disulfide bridges
- Ionic bonds
- Hydrogen bonds
Describe disulfide bonds in tertiary structure of proteins
Strong covalent S-S bonds between molecules of the amino acid cysteine
Describe ionic bonds in the tertiary structure of proteins
Relatively strong bonds between charged R groups (pH changes causes these bonds to break)
Describe hydrogen bonds in tertiary structure of proteins
Numerous and easily broken
Define 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 groups