The Michaelis-Menton model:
Is based on the assumption that a transition state is formed in the enzyme active site.
Is based on the assumption that an enzyme catalysed reaction is mediated at the active site of an enzyme.
Is based on the assumption that a biochemical reaction is at equilibrium.
Is based on the assumption that a biochemical reaction occurs at a steady state.
The Michaelis-Menton constant, Km:
Is associated with the maximum enzyme activity observed when the all active sites in an enzyme are saturated with substrate.
Is associated with the number of substrate molecules reacted on by an enzyme molecule per unit time.
Is associated with the affinity of an enzyme for a specific substrate.
Is associated with the selectivity of an enzyme for different substrates.
In enzyme kinetics, the ratio of constants kcat/Km:
Is a measure of the rate of acceleration carried out by the enzyme.
For a given enzyme is independent of the substrate used.
Has units of concentration.
Gives an idea of the enzymes catalytic efficiency.
The Km/Kcat ratio:
In enzyme catalysis, the term ‘approximation’ refers to:
A catalytic strategy facilitating transition state formation through covalent bond formation between the substrate and enzyme active site.
A catalytic strategy facilitating transition state formation through hydrogen bond formation and electrostatic bond formation between the substrate and enzyme active site.
A catalytic strategy facilitating transition state formation through interaction involving metal ions and substrate in the enzyme active site.
A catalytic strategy facilitating transition state formation through direct transfer of a proton to or from the substrate in the enzyme active site.
Consider an enzyme that shows Michaelis-Menten kinetics where: v0 = Vmax . [S] / (Km + [S]) If a substrate, S, is present at a concentration of 8 mM, and Km is 4 mM, the rate of reaction (v0) measured will be:
Half of Vmax
Two thirds of Vmax
Double Vmax
Three times Vmax
Koshland’s induced fit model for enzyme-substrate complex formation:
May explain why enzymes have particular substrate specificity.
May explain why enzymes are able to catalyse chemical reactions that cannot be facilitated in any other way
May explain why enzymes increase the rate of a reaction by reduction of the activation energy change for the reaction
May explain why enzymes can effectively reduce the loss of energy from a chemical reaction as heat
Enzymes:
are chemically altered at the end of their reaction
are involved in changing the equilibrium constant of the reaction that they catalyse
bind their substrates at their active site(s)
increase the activation energy of the reaction they catalyse
The Michaelis constant, Km:
Is a measure of the rate acceleration caused by the enzyme
For a given enzyme is independent of the substrate used
Has units of concentration
Gives an idea of the enzyme’s catalytic efficiency
The Vmax of an enzyme catalysed reaction:
Is altered when a competitive inhibitor is present
Can be determined from the intercept on the x-axis of a Lineweaver-Burk plot
Is the maximum rate at which the enzyme can convert substrate into product
Proteosome-mediated proteolysis:
Is controlled by serine protease enzymes.
Is a key part of the control mechanism in the eukaryote cell cycle
Is a key part of the control mechanism in the prokaryote cell cycle
Is controlled by ubiquinone activating enzymes.
The Alanine Cycle:
Is completely located in the mitochondrial matrix.
Facilitates transport of ammonia produced in the liver to the muscles where it can be used in anabolic processes - preventing the exposure of free ammonium to other components of eukaryote tissues.
Facilitates transport of ammonia produced in the muscles to the liver where it can be effectively removed from the body - preventing the exposure of free ammonium to other components of eukaryote tissues.
Is located in the cell membrane of muscle cells.
The transition state for an enzyme-catalysed reaction:
Describes the way the substrate interacts with the enzyme.
Descibes the protein tertiary structure when the enzyme substrate is converted to a product.
Describes the form the substrate takes that facilitates the formation of a low energy intermediate during the catalytic cycle.
Describes an intermediate in the catalytic cycle that is produced in order to minimise the activation energy for the reaction.
An enzyme has the following kinetic parameters: Km = 20mM, Vmax= 50 mM.s-1 Using the equation: v0=Vmax.[S]/Km+[S] When the rate, v0, is measured at 30 mM.s-1; the substrate concentration, [S] will be:
15 mM
30 mM
45 mM
60 mM
The value of ΔG0’ for an enzyme catalysed reaction:
Will always be negative if an enzyme catalysed reaction proceeds spontaneously.
Will always be positive if an enzyme catalysed reaction proceeds spontaneously.
Will always be equal to the ΔG value for a reaction where both the reactants and products have an equal concentration.
Will only apply to a reaction occurring if the pH = 7.0 in aqueous solution.
An abzyme is:
An enzyme that is protein-engineered to work like an antibody.
An antibody that is protein-engineered to work like an enzyme.
An enzyme that has high affinity for a transition state analogue.
An antibody that has high affinity for a transition state analogue.
An oxyanion hole is:
A region of the enzyme active site that facilitates binding of positively charged substrates through their association with oxygen-containing amino-acid side chains in the enzyme.
A region of the enzyme active site that facilitates binding of negatively charged substrates through their association with oxygen-containing amino- acid side chains in the enzyme.
A region of the active site that facilitates binding of positively charged oxygen- containing groups present in a substrate.
A region of the active site that facilitates binding of negatively charged oxygen-containing groups present in a substrate.
Which of the following catalytic strategies is not employed by the enzyme chymotrypsin:
Approximation.
Acid-base catalysis.
Metal-ion catalysis.
Covalent catalysis.
The urea cycle:
Is completely located in the mitochondrial matrix – preventing the exposure of free ammonium to other components of the eukaryote cell.
Allows free ammonia obtained directly from deamination of glutamate to be converted to urea – preventing the exposure of free ammonium to other components of the eukaryote cell.
Allows free ammonia obtained directly from deamination of tryptophan to be converted to urea – preventing the exposure of free ammonium to other components of the eukaryote cell.
Is completely located in the cytoplasm of the cell – preventing the exposure of free ammonium to other components of the eukaryote cell.
An end-product can act to inhibit an enzyme by binding at the:
Active site
Activation site
Allosteric site
Transitional site
Serine proteases:
Are proteases that hydrolyse polypeptides with serine in the F1 position
Are proteases that are found in the cytoplasm of all cells
Utilise a serine residue at the active site to facilitate substrate binding
Utilise a serine residue at the active site to facilitate cleavage of peptide bonds
In acid-base catalysis:
An acidic- or basic- amino acid in the active site of an enzyme facilitates transition state formation by hydrogen abstraction from an appropriate substrate.
An acid- or basic- substrate in the active site of an enzyme facilitates transition state formation by hydrogen abstraction from a catalytic amino acid in the active site.
Both are correct.
Neither are correct.
The protein ubiquitin:
Can be covalently linked to proteins via the N-terminus glycine residue.
Is a polypeptide.
Is an essential component of eukaryote respiratory chains.
Can be covalently linked to proteins via isopeptide bond formation.
If the ΔG°′ of the reaction Malate → Oxaloacetate is +30 kJ/mol, what will happen in the presence of malate dehydrogenase under standard conditions?
The reaction will proceed fast with the formation of the explosive products.
The reaction will not occur spontaneously.
The reaction will never reach equilibrium.
The reaction will proceed spontaneously from left to right.
THE ENZYME-SUBSTRATE COMPLEX:
Is a key concept that helps to explain how enzymes reduce activation energy for chemical reactions.
Is a key concept that helps to explain how enzymes can reduce the Gibb’s free energy for a chemical reaction.
Is a key concept that helps to explain how enzymes can exhibit diverse substrate specificity.
Is a key concept that helps to explain how enzymes may exhibit Michaelis-Menton kinetics.
THE ENTHALPY CHANGE ASSOCIATED WITH A BIOCHEMICAL REACTION:
Is a term used to describe the amount of randomness or disorder that results as the reaction proceeds
Is a term used to describe the amount of ‘free energy’ change that results as the reaction proceeds
Is a term used to describe the amount of heat that is produced or consumed as the reaction proceeds
Is always determined at room temperature (25oC)
ENZYMES USUALLY UTILISE ONE OR MORE TRANSITION METAL ATOMS AT THE ACTIVE SITE TO:
Facilitate substrate binding
Facilitate transition state formation
Facilitate stabilisation of the tertiary structure
Facilitate conformational changes in the protein during the catalytic cycle
ENZYMES:
Reduce the entropy associated with chemical reactions
Reduce the enthalpy associated with chemical reactions
Reduce the Gibb’s free energy associated with chemical reactions
Reduce the activation energy associated with chemical reactions
CONSIDER TWO REACTIONS. REACTION 1 HAS A ΔG°′ VALUE OF -20 kJ.mol-1 AND REACTION 2 HAS A ΔG°′ VALUE OF -50 kJ.mol-1. WHICH REACTION PROCEEDS AT THE FASTEST RATE AT ROOM TEMPERATURE AND PRESSURE AND pH 7?
Reaction 1
Reaction 2
They both occur at much the same rate
It is not possible to know this from the data provided
THE CATALYTIC EFFICIENCY OF AN ENZYME CATALYSED REACTION:
Can be described by the ratio: kCAT/KM
Can be described by the ratio: KM/kCAT
Can be described by the ratio: Vmax/kCAT
Can be described by the ratio: kCAT/Vmax
WHEN CONSIDERING ENZYME CATALYTIC MECHANISMS, ACID-BASE CATALYSIS IS USUALLY DEPENDANT UPON:
Hydrogen bonding with at least one amino acid side chain at the active site to facilitate formation of the transition state.
Hydrogen bonding between the carbonyl and amide groups of peptide bonds to facilitate formation of the transition state.
Hydrogen bonding between a water molecule and the substrate to facilitate formation of the transition state.
Hydrogen bonding with an oxidised metal ion prosthetic group in the active site to facilitate formation of the transition state.
IF THE ΔG°' OF THE REACTION A → B is –20 kJ/mol, WHAT WILL HAPPEN IN THE PRESENCE OF A SPECIFIC ENZYME UNDER STANDARD CONDITIONS?
The reaction will stop
The reaction will proceed spontaneously from B to A
The reaction will proceed spontaneously from A to B
The reaction will not occur spontaneously
FOR THE FOLLOWING REACTION: L-Malate + NAD+ → Oxaloacetate + NADH + H+ ΔG°' = +29.7 kJ/mol. WHICH OF THE FOLLOWING STATEMENTS IS CORRECT?
This reaction can only occur in a cell in which NADH is converted to NAD+ by the respiratory chain
This reaction can only occur in a cell if it is coupled to another reaction for which ΔG°' is large and negative
This reaction may occur in cells at some concentrations of substrate and product
This reaction is energy-releasing
IN MICHAELIS-MENTON KINETICS, FORMATION OF THE ENZYME-SUBSTRATE COMPLEX:
Is always the rate limiting step in an enzyme catalysed reaction
Is never the rate limiting step in an enzyme catalysed reaction
Is always a necessary pre-requisite to formation of the transition state and therefore product turnover
Is never a necessary pre-requisite to formation of the transition state and therefore product turnover
COMPETITIVE INHIBITORS:
alter the Vmax of the reaction
show irreversible binding to their target enzyme
resemble the structure of the natural substrate/product molecule
bind at a site distant from the active site
MULTIPLICATION OF UBIQUITIN TAGGING:
Inhibits proteosome-mediated protein degradation
Is essential for proteosome-mediated protein degradation
Enhances proteosome-mediated protein degradation
Has nothing to do with proteosome-mediated protein degradation
