Proteins Flashcards
What is the general structure of an amino acid?
- COOH carboxyl group
- R variable side group consists of carbon chain & may include other functional groups e.g. benzene ring or -OH (alcohol)
- NH2 amino group
Describe how to test for proteins in a sample.
Biuret test confirms presence of peptide bond.
1. Add equal volume of sodium hydroxide to sample at room temp
2. Add drops of dilute copper (II) sulphate solution. Swirl to mix.
3. 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 R group
How do dipeptides and polypeptides form?
- Condensation reaction forms peptide bonds & eliminates molecule of water
How many levels of protein structure are there?
4
Define primary structure of a protein.
- Sequence, number & 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 (slightly -ve) attached to -C=O & H (slightly +ve) attached to -NH
Describe the 2 types of secondary protein structure
a-helix:
- all N-H bonds on same side of protein chain
- spiral shape
- H-bonds parallel to helical axis
B-pleated sheet
- N-H & C=O groups alternate from one side to the other
Define tertiary structure of a protein. Name the bonds present.
3D structure formed by further folding of polypeptide
- disulphide bridges
- ionic bonds
- hydrogen bonds
Describe each type of bond in the tertiary structure of proteins.
- Disulphide bridges: strong covalent S-S bonds between molecules of the amino acid cysteine
- Ionic bonds: relatively strong bonds between charged R groups (p changes cause these bonds to break)
- Hydrogen bonds: numerous & 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 bonds as tertiary structure
- May involve addition of prosthetic groups e.g., metal ions or phosphate groups
Describe the structure and function of globular proteins.
- Spherical & compact
- Hydrophilic R groups face outwards & hydrophobic R groups face inwards = usually water-soluble
- Involved in metabolic processes e.g., enzymes & haemoglobin
Describe the structure and function of fibrous proteins.
- Can form long chains of 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 the capillary tube to spot mixture onto pencil origin line & place chromatography paper in solvent.
- Allow solvent to run until it almost touches the other end of the 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.
- Calculate Rf values & match to database.
What are enzymes?
- Biological catalysts for intra & extracellular reactions.
- Specific tertiary structure determines shape of active site, complementary to a specific substrate.
- Formation of enzyme-substrate complexes lowers activation energy of metabolic reactions.
