Proteins Flashcards
What is the general structure of an amino acid?
- COOH carboxyl/carboxylic acid group
- R variable side group
- NH2 amine/amino group
- Central carbon atom
- A hydrogen atom
Describe 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 (1 & 2 make biuret reagent)
- Results:
- Positive result: colour changes from blue to purple
- Negative result: solution remains blue
How many amino acids are there and how do they differ from one another?
- 20
- Differ only by one side ‘R’ group
How do dipeptides and polypeptides form?
- Condensation reaction forms peptide bond (-CONH-) & eliminates molecule of water
- Dipeptide: 2 amino acids
- Polypeptide: 3 or more amino acids
How many level 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, and determines the proteins function in the end
Define ‘secondary structure’ of a protein
- The shape that the chain of amino acids chains – either alpha
helix or beta pleated sheet - The hydrogen in the -NH has a slight positive charge whilst
the oxygen in the -C=O has a slight negative charge - As a result weak hydrogen bonds can form leading to alpha helices or beta pleated sheets
Describe the 2 types of secondary protein structure
α-helix:
- All N-H bonds on same side of protein chain
- Spiral shape
- H-bonds parallel to helical axis
β-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
- Disulfide bridges
- Ionic bonds
- Hydrogen bonds
Describe each type of 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 & 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 (haemoglobin) 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 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 & 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 & 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 (ES) complexes lowers activation energy of metabolic reactions
