3.1.4 Proteins and Enzymes Flashcards
Draw the structure of an amino acid
See notes
A polymer on amino acids
Polypeptide
Describe how a dipeptide forms
- Condensation reactions
- Between carboxyl and amino group
- Forms a peptide bond
Primary sequence of protein
Number, sequence, order
Secondary structure of protein
- Hydrogen bonds between carbons and amino group
- Causes a long polypeptide chain to twist in a 3D shape (coil)
- Forms alpha helix or beta pleated sheet
Tertiary structure of proteins
- R groups in polypeptide chain determine how it folds into specific 3D shape
- Forms ionic, hydrogen or disulfide bonds
Quaternary structure of proteins
Joining of 2 polypeptide chains
Describe how to test for proteins
- Biurets reagent
- Blue to purple
Lyxose is a sugar which binds to enzymes. It is not involved in inhibition.
Explain why lyxose increases the rate of reaction of enzymes (4)
- Lyxose changes the tertiary structure
- This induces a change in the shape of the active site
- Making it more complementary
- Allowing more enzyme substrate complexes to form.
Explain how enzymes increase the rate of reaction
They lower the activation energy by putting pressure and bending bonds allowing reactions to take place at a lower temperature
3 conditions needed for enzyme action
- Enzymes and substrates must collide with sufficient energy
- Requires activation energy
- Free energy of products must be lower than reactants
Describe the structure of enzymes (2)
Any 2 from
- Globular proteins with a specific 3D shape
- Contain an active site
- The active site is complementary allowing only one substrate to bind
- Enzyme substrate complexes can form
Describe the difference between the lock and key model and induced fit model of enzyme action (2)
- Lock and key model does not take into account that the active site is not rigid/fixed/only complementary to one substrate
- The induced fit model causes the active site to induce a change in shape (making it more complementary)
Describe the induced fit model of enzyme action (4)
- Substrate is not complementary (to the active site)
- Substrate enters the active site and induces a change in shape of the active site, so, an enzyme substrate complexes forms
- This stresses the bonds lowering activation energy
- When products leave, the active site returns to it’s original shape
How can enzyme catalysed reactions be measured?
- Formation of products
- Disappearance of substrate
Explain what would be seen in an enzyme catalysed reaction (5)
- At first, lots of substrate but little product, substrate can easily make contact with empty active sites
- Active sites are filled and substrate is rapidly broken down into products
- Amount of substrate decreases, formation of product increases
- Becomes difficult for substrate molecules to to contact the active site as products begin to obstruct it
- Rate of reaction decreases, causing the graph to tail off
If no other mark awarded, allow:
- Graph flattens as all the substrate has been used up
The effect of temperature on enzyme action
- Temperature increase: increased kinetic energy, higher frequency of collisions, more enzyme substrate complexes form, rate of reaction increases
- However, too high and it could causes hydrogen and ionic bonds between R groups of amino acids to break, so tertiary structure changes causing the active site to change shape, this denatures the enzyme
State the definition of ‘denatured’
Permanent change, so the enzyme will never function again
The effect of pH on enzyme action
- Alters charge of amino acids that form active site, substrate can’t bind, and enzyme substrate complexes cannot form
- Depending on how significant the change is, may cause bonds maintaining tertiary structure to break
What does a buffer allow?
pH to remain constant
How might isolation change an enzymes activity
Outside the optimum conditions of the host cell
If substrate is limiting, how will this affect enzyme activity?
Increasing enzyme concentration will have no effect
Explain how substrate concentration affects rate of enzyme action (enzyme has a fixed concentration)
- As substrate concentration increases, the rate of reaction increases in proportion to concentration of substrate, because at low substrate concentrations, enzyme molecules have limited number of substrate molecules to collide with
- As more substrate is added, active sites gradually fill, until they work at fastest rate, after this, adding more substrate has no effect
Explain how a competitive inhibitor decreases the rate of reaction (3)
- Have a similar shape to the substrate
- (So) bind to the active site
- Preventing enzyme substrate complexes forming
How can a competitive inhibitor be overcome?
Increasing the substrate concentration
Explain how a non competitive inhibitor decreases the rate of enzyme action (3)
- Bind away from the active site
allow allosteric site - Alter/change the shape of the active site so it is no longer complementary
- Prevents enzyme substrate complexes forming
Explain how enzymes with different amino acid sequences can catalyse the same reaction (2)
- Both have similar tertiary structures
- So can form enzyme substrate complexes
List the four factors affecting enzyme controlled reactions (4)
- Temperature (ignore heat)
- pH
- Substrate concentration
- Enzyme concentration
Accept in any order
Ignore any reference to volume/amount/mass of substrate/enzyme
Scientists investigated the effect of pH 8.4 and pH 7.5 on the activity of enzymes P and Q.
Describe what the scientists should place in the control tubes in this investigation (3)
- Same volume of (each) buffer/pH solution;
- Same concentration/mass of substrate (at start);
- Same concentration/mass of denatured enzyme;
Accept description of denatured eg boiled
If no marks gained, accept for 1 mark,
Buffer and substrate and denatured enzyme
OR
Buffer and substrate and no enzyme
OR
Buffer and substrate and water
*Ignore temperature, amount for volume, concentration OR mass*
*Accept pH solution for buffer*
Cyclin D stimulates the phosphorylation of DNA polymerase, which activates the DNA polymerase.
Describe how an enzyme can be phosphorylated (2)
- Attachment/association of (inorganic) phosphate (to the enzyme);
- (Released from) hydrolysis of ATP
Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule (2)
- Condensation (reaction)/loss of water;
- (Between) phosphate and deoxyribose
- (Catalysed by) DNA polymerase
Explain how the active site of an enzyme causes a high rate of reaction (2)
- Lowers activation energy;
- Induced fit causes active site (of enzyme) to change shape;
- (So) enzyme-substrate complex causes bonds to form/break
Explain how two enzymes with different amino acid sequences can catalyse the same reaction (2)
- (Both) active sites have similar/identical tertiary structures
OR
(Both) active sites have identical amino acid sequences;
Ignore shape for tertiary structure
Accept (both) have active sites that are complementary to different parts of the substrate;
Accept attach/bind for complementary
- (So) form enzyme-substrate complexes (with the same substrate);
Accept E-S for enzyme-substrate
A student prepared an experiment with 2 enzymes at different pH’s.
Describe what the student should place in a control tube for this investigation (3)
- Same volume of (each) buffer/pH solution;
- Same concentration/mass of substrate (at start);
- Same concentration/mass of denatured enzyme;
Accept description of denatured, eg boiled
Describe how monomers join to form the primary structure of a protein (3)
- Condensation reaction between amino acids;
Accept descriptions of condensation reaction: eg loss of water - (Forming) peptide bonds;
- Creating (specific) sequence/order of amino acids;
Describe one similarity and one difference between the induced-fit model of enzyme action and the lock and key model of enzyme action (2)
(Similarity)
- Substrate fits/binds to active site
OR
Enzyme-substrate complex (formed);
(Difference)
- Active site changes shape, but does not change in lock and key
OR
(Initially) active site not complementary to substrate with induced-fit, but is complementary in lock and key;
Reject ‘substrate changes shape’
Accept ‘flexible’ for changes shape and ‘rigid’ for does not change
Describe how a quarternary protein is formed from its monomers.
Do not include the process of translation in your answer (5)
- Amino acids joined by peptide bond(s);
- (By) condensation reaction(s);
- Secondary structure is formed by hydrogen
bonding; - Tertiary structure formed by interactions
(between R groups); - Quaternary structure contains >1 polypeptide
OR
Quaternary structure formed by
interactions/bonds between polypeptides; - Accept alpha helix
OR β-pleated sheet
for ‘secondary
structure’ - Accept 3o for
tertiary - and 5. Accept for
‘interactions’,
hydrogen bonds OR
disulfide bridges OR
ionic bonds OR
hydrophobic OR
hydrophilic
interactions - Ignore peptide
Describe the primary structure of all proteins (2)
- Sequence/order of amino acids;
- (Joined by) peptide bonds;
