Enzymes II Flashcards
How do enzymes catalyze reactions?
- Conversion of reactants to products, reacting molecules must overcome energy barrier.
- Enzymes facilitate reactions by reducing the energy barrier.
- E enhance reactions by binding with S to form complex with a lower ΔG*:
E + S <==> ES –> E + P
What are the theories about how enzymes work?
o Collision theory
o Transition state theory
What is a transition state theory?
the free energy in this state is much higher than the free energy in the substrate free energy, finally the new bond will form to make new products.
For a reaction to happen, there should be enough energy to carry the substrate to the transition state (activation energy)
The free energy that the substrate need to get to the transition state is called the _______
ACTIVATION ENERGY
_____ is the minimum amount of energy required to overcome the energy barrier to get to the activated complex of the transition state.
DELTA G
_____ symbolizes the minimum amount of energy required to overcome the energy barrier to get to the activated complex of the transition state.
DELTA G
True/False
The larger the activation energy is the more difficult for the reaction to occur
True
Enzymes as a type of catalysis accelerate chemical reaction by lowering the _______.
activation energy
The collision theory
If molecules move too slowly with little kinetic energy, or collide with improper orientation, they do not react and simply bounce off each other. However, if the molecules are moving fast enough with a proper collision orientation, such that the kinetic energy upon collision is greater than the minimum energy barrier, then a reaction occurs
How do enzymes lower the activation of energy?
Enzyme can transiently bind with the substrate to produce a transition state “E-S” having a lower activation of energy than the transition state of the uncatalyzed reaction
How can we measure the enzyme activity?
Reaction rate of enzymatic reaction
How can we measure the reaction rate?
The rate of disappearance of substrate or
The rate of the formation of product (normally this is used bc it is quite reliable)
Give examples for the initial rate experiments
continuous method
discontinuous method
Continuous Method for initial rate
∇ with the catalysis taking place, measure or calculate the Rx rate within the linear period
Discontinuous method for initial rate
∇ Endpoint method ∇ Stop the reaction ∇ After the catalysis takes place for some time, some chemicals such as acids, alkali, inhibitors are added to stop the reaction ∇ The reaction rate can be calculated Rx = D(P)/D(t)
Advantages of using continuous method
- Easy to find the linear range
- Enzyme is active
- Can finish in a short time
Advantages of using discontinuous method
Cheap equipment
Easy to perform
Disadvantages of using continuous method
Equipment is expensive
Needs good experiments skills
Disadvantages of using discontinuous method
- Maybe beyond the linear range
- Enzyme is denatured or inhibited
- Time consuming
In the lab I we used the _______ method to determine the activity of peroxidase
continuous
What are the factors that can affect enzyme catalysis?
o Temperature o PH. o Enzyme concentration o Substrate concentration o Activator o Ions or ionic strength o Solvents
How does an increase in substrate affect the rate of the reaction?
∇ In general, increase of substrate concentration the catalysis will increase up to a certain point until the enzyme is saturated.
What is Km (Michealis-Menton constant)?
it is the affinity of E to S
Lower Km means higher affinity
The Km or Michaelis constant is the substrate concentration at which v = Vmax/2 and its usual unit is M.
True/False
Lower Km means higher affinity
true
What is the equation that expresses the steady state of a reaction?
E + S –> (k1,k2) ES –> (k3) E + P
K1 (E)(S) = K2 (ES) + K3(ES)
(E)(S)/(ES) = (K2 + K3)/K1 = KM
What is Michealis-Menton equation?
V = (VMAX)(S)/(Km + (S))
When V = Vmax/2 the Km equals to ____
(Vmax)(S)/(KM + (S)) =
In value,
KM = (S)
when V=Vmax/2
Vmax symbolizes the ______
the ease of transformation of S to E
How is the efficiency of catalysis expressed?
Vmax/Km means the catalysis efficiency
What does a M-M graph look like?
rectangular hyperbola
What does Vmax represent?
The Vmax is the maximum rate of the enzyme-catalysed reaction and it is observed at very high substrate concentrations where all the enzyme molecules are saturated with substrate, in the form of ES complex.
Express Vmax in terms of Kcat and [Et]
Vmax = kcat[Et]
where [Et] is the total enzyme concentration and kcat is the rate of breakdown of the ES complex (k+2 in the equation), which is known as the turnover number.
What does Kcat represent?
kcat represents the maximum number of substrate molecules that the enzyme can convert to product in a set time.
What does Km depend on?
The Km depends on the particular enzyme and substrate being used and on the temperature, pH, ionic strength, etc.
True/False
Km is independent of the enzyme concentration, whereas Vmax is proportional to enzyme concentration.
True
Lineweaver-Burke Method
A better method for determining the values of Vmax and Km was formulated by Hans Lineweaver and Dean Burk and is termed the Lineweaver- Burk (LB) or double reciprocal plot. Specifically, it is a plot of 1/v versus 1/[S], according to the equation:
1/v = (Km/ Vmax)*(1/S)+(1/Vmax)
The slope of the Linewaever-Burke Method
a straight line with a slope of Km/Vmax.
How does temperature effect enzyme catalysis? What happens to the enzymatic activity in every 10’C increase?
The rate of an enzymatic reaction increases with increasing temperature. Although there are significant variations from one enzyme to another, on average, for each 10◦C rise in temperature, the enzymatic activity is increased by an order of two.
At what temperature does denaturation occur in general?
fter exposure of the enzyme to high temperatures (normally greater that 65◦C), denaturation of the enzyme may occur and the enzyme activity decreased.
What is Hane’s Method?
is a graphical representation of enzyme kinetics in which the ratio of the initial substrate concentration [S] to the reaction velocity V is plotted against [S].
1/V = Km/Vmax * 1/(S) + 1/Vmax (S)/V = Km/Vmax + (S)/Vmax
What is the slope, y-intercept and x-intercept of Hane’s Method plot?
perfect data will yield a straight line of slope 1/Vmax, a y-intercept of Km/Vmax and an x-intercept of −Km.
What Eadie-Hofstee’s Method?
is a graphical representation of enzyme kinetics in which reaction rate is plotted as a function of the ratio between rate and substrate concentration:
v=(-Km)*(v/S) + Vmax
where v represents reaction rate, KM is the Michaelis–Menten constant, [S] is the substrate concentration, and Vmax is the maximum reaction rate.
Which represents the affinity of E for S?
Km
What repents the ease of catalysis?
Vmax
What represents the catalytic efficiency?
Vmax/Km
What is enzyme inhibition?
• Reduction in rates of enzyme-catalysis caused by inhibitors
What are the two types of inhibition?
Inhibition may be reversible or irreversible
- E + I ==> EI ; or
- E + I <===> EI
What are the types of reversible inhibition?
- competitive
- non-competitive
- uncompetitive
What is Competitive Inhibition?
- Competitive Inhibition:- “I” binds to active site of “E”, and prevents “S” from binding;
- “I” resembles “S” in structure, thus competes for active site;
- e.g., => inhibition of succinate dehydrogenase by malonate.
True/False
In competitive inhibition the inhibitor resembles the substrate in structure, thus competes for active site.
true
Give an example for competitive inhibition
- inhibition of succinate dehydrogenase by malonate.
- natural compounds as competitive inhibitors for lipase, glycosidase for the control of obesity, diabetes
What is Non-competitive Inhibition?
“I” binds to a site other than the active site of “E”, and prevents transformation of “S” to “P”;
Reduction of Rx rate by pH is an example of NCI - the “I” being the H+ on the acid side of the optimum, and OH- on the alkaline side.
Give an example for non-competitive inhibition
Reduction of Rx rate by pH is an example of NCI - the “I” being the H+ on the acid side of the optimum, and OH- on the alkaline side.
Dilute metal salts, dilute acid, dilute alkali. etc. as non-competitive inhibitors
True/False
In non-competitive inhibition the inhibitor binds to the active site.
False
“I” binds to a site other than the active site of “E”, and prevents transformation of “S” to “P”
What can be a competitive-inhibitor?
Dilute metal salts, dilute acid, dilute alkali. etc. as non-competitive inhibitors
What is uncompetitive Inhibition?
“I” binds to site on “E” molecule which becomes available only after S has bound to the active site, i.e., to the ES-complex;
• E.g., => S inhibition which occurs at very high [S]. As in inhibition of invertase by high conc. of sucrose.
True/False
Uncompetitive inhibitor binds to the substrate
False
“I” binds to site on “E” molecule which becomes available only after S has bound to the active site, i.e., to the ES-complex;
Give an example for uncompetitive inhibition
♣ Sucrose (invertase (E.C. XXX) + H2O) glucose + fructose
♣ Inhibitor binds to site on enzyme molecule which becomes available only after substrate has bound to the active site, to the E-S complex,
♣ High (sucrose) act as an un-competitive inhibitor
♣ This reaction used in confectionary industry, since fructose is sweeter and not easy to crystallize
How to relieve competitive inhibition?
increase (S)
How to relieve non-competitive inhibition?
removal of inhibitors using separation techniques dialysis, ultrafiltration
How to relieve uncompetitive inhibition?
dilute (S) or add (E)
The factors affected by temperature on enzyme catalysis
(a) Enzyme stability
(b) Affinity of Enzyme for Substrate; Inhibitors or
Activators
(c) rate of conversion of Substrates to Products (d) changes in solubility of gases
(e) pH of buffer
(f) competing reactions
(g) ionization of prototropic groups
Common effects of temperature on enzyme catalysis
- Low temperatures slow down enzyme catalysis (used in foods to retard undesirable effects like texture softening, off-flavors, ripening, etc);
- Higher temperatures enhance Enzyme catalysis, but may also deactivate Enzymes.
What is the Q10 equation?
As the temp. increases, the enzyme catalysis increases.
Q10 – the reaction rate can be doubled every 10 degrees.
Q10 = Rx rate @ T + 10C / Rx rate @ TC
What is the shape of the graphical representation of “Enzyme Activity” Vs. “Temperature”?
bell-shaped curve
• Activation Topt
Fill in the blanks: inactivation/activation
• ______ Topt
Activation
Inactivation
What is Arrhenius Equation?
provides a quantitative description of the relationship between the rate of an enzyme-catalysed reaction (Vmax) and the temperature (T).
Where Ea is the activation energy of the reaction, R is the gas constant, and A is a constant relevant to the nature of the reactant molecules.
k = Ae^-Ea/RT
Ln k = LnA - Ea/RT
Graphical presentation of Ln k Vs. 1/T
Influence of PH on enzyme catalysis
E’s active within narrow pH range, pH optimum (optima) of enzyme(s) - duration of Rx - nature of S , & [S] - ionic strength of medium - purity of E - presence / absence of Inh or Act
o pH. Optimum within pH. Value 5-8
o exception is pepsin = pH. 1.5
o papain has a broad optimum range compared to other enzymes.
_______ has a broad optimum range compared to other enzymes
papain
