proteins Flashcards
Describe the general structure of an amino acid
- Contains a COOH group
- Contains an R variable group
- Contains an NH2 amino group
Describe a method to test for proteins in a sample
- Add equal volumes of dilute sodium hydroxide (NaOH) to a sample at room temperature
- Add drops of dilute copper (II) sulfate solution (CuSO4), swirl to mix
- A negative result = solution remains blue, a positive result is when the colour changes from blue to purple
How many amino acids are there and how do these differ from each other
20
differ by side ‘R’ group
Explain how do dipeptides and polypeptides form?
- Condensation reaction forms a peptide bond between (-CONH-) and eliminates a molecule of water
- Dipeptides are formed from 2 amino acids
- Polypeptides are formed from 3 or more amino acids
How many levels of protein structure are there
4
Describe the primary structure of a protein
- Sequence, number and type of amino acids in the polypeptide
- Determined by a sequence of codons on mRNA
Describe the secondary structure of a protein
- Hydrogen bonds form between negatively charged nitrogen, oxygen and positively charged hydrogen
- Causes polypeptide chain to fold into an alpha helix or a beta pleated sheet
Describe the tertiary structure of a protein
- 3D structure formed by further folding of polypeptide due to: disulfide bridges, ionic bonds or hydrogen bonds
Describe disulphide bridges in the tertiary structure of proteins
Strong covalent S-S bonds between molecules of amino acid cysteine
Describe ionic bonds in the tertiary structure of proteins
Strong bonds between charged R groups
Describe hydrogen bonds in the tertiary structure of proteins
Numerous and easily broken
Describe the quaternary structure of a protein
- Forms when there is more than one polypeptide chain
- Held together by disulphide bridges, hydrogen bonds or ionic bonds to form an alpha helix and a beta-pleated sheet
Describe the structure and function of globular protein
- Spherical and compact
- Hydrophilic R groups face outwards and hydrophobic R groups face inwards so are water soluble
- Involved in metabolic processes
Describe the structure and function of fibrous protein
- Can form long chains or fibres
- Insoluble in water
- Useful for structure and support
Define enzymes
Biological catalysts for intra and extracellular reactions
What determines the shape of an enzyme active site
The tertirary structure
How is the activation energy of metabolic reactions lowered
The formation fo enzyme-substrate complexes lowers the activation energy of metabolic reactions
Explain the induced fit model of enzyme action
- The enzyme active site isn’t completely complementary to the substrate
- Active site shape changes as substrate binds and enzymes-substrate complex forms in an ideal binding agreement
- This stresses/distorts the bonds in the substrate which maximises the ability of the enzyme to catalyse the reaction
Explain the lock and key model of enzyme action
- The active site is a rigid, fixed shape
- The active site is completely complementary to 1 substrate
- After a collision, enzyme-substrate complex forms leading to a reaction.
Name 5 factors that affect the rate of enzyme-controlled reactions
- Enzyme concentration
- Substrate concentration
- Concentration of inhibitors
- pH
- Temperature
How does substrate concentration affect the rate of an enzyme-controlled reaction?
- As the number of substrate molecules increases, the likelihood of enzyme-substrate complex formation increases
- If the enzyme concentration remains fixed but the amount of substrate is increased, all active sites will become saturated and the substrate concentration will not increase the reaction rate
- When the active sites of enzymes are all saturated, any substrate molecules that are added will have nowhere to bind in order to form an enzyme-substrate complex
How does enzyme concentration affect the rate of an enzyme-controlled reaction?
- The higher the enzyme concentration in a reaction mixture, the greater the number of active sites available and the greater the likelihood of enzyme-substrate complex formation
How does a low temperature affect the rate of enzyme-controlled reactions
- Molecules move relatively slow and lower the frequency of successful collisions between substrates
- There are less frequent enzyme-substrate complex formations.
- Substrate and enzymes collide with less energy, making it less likely for bonds to be formed or broken which stops the reaction from occurring.
How does a high temperature affect the rate of enzyme-controlled reaction
- Molecules move more quickly and there is a higher frequency of successful collisions between substrate molecules and the active site of enzymes. There are more frequent enzyme-substrate complex formations.
- Substrate and enzyme collide with more energy, making it more likely for bonds to be formed or broken which allows the reaction to occur.
How does pH affect the rate of enzyme-controlled reaction
- Enzymes are denatured at extremes of pH.
- The hydrogen and ionic bonds that hold the tertiary structure of the protein together can break
- Altering the shape of the active site so enzyme-substrate complexes form less easily
Compare competitive and non-competitive inhibitors
- Competitive inhibitors have a similar shape to the substrate so bind to the active site, non-competitive inhibitors bind at the allosteric binding site
- Competitive inhibitors do not stop reactions, enzyme-substrate complexes form when an inhibitor is released, and non-competitive stop reactions because they trigger to change in shape
- Competitive inhibitors = increasing substrate concentration decreases their effect, non-competitive inhibitors = increasing substrate concentration has no impact on their effect
How can you reduce the effect of the competitive inhibitor?
Increasing substrate concentration reduces the effect of the inhibitor as the level of inhibition is dependent on relative concentrations.
Why are enzymes specific?
- The shape of the active site (and its specificity) is determined by the ternary structure of the protein that makes up the enzymes
