Enzyme Kinetics Flashcards
Catalyst:
substance that increases the rate of a chemical reaction without being consumed in the reaction
provides an alternative pathway for the reaction in which the rate-determining step has a lower Gibbs activation energy than that of the non-catalysed reaction
Catalysis
- Does not appear in the overall equation for the reaction although it may appear in the rate equation
- Some substances slow down chemical reactions and these are often called negative catalysts or inhibitors
- Does not affect the amount of product formedβonly the rate of its formation
- Small amount is usually needed
- Not changed chemically in the reaction and can be recovered at the end
- Sometimes may be changed physically
At a given temperature, the rate constant is greater for the catalysed reaction, so the reaction is ______
faster
Non-catalysed reaction proceeds through a _____ _______ state, whereas the mechanism for the catalysed reaction involves the formation of an intermediate
single transition
Enzyme-Catalysed reactions
Enzymes catalyse a wide range of reactions in biochemical systems
(human body dilute solution at 37 Β°C). Without the presence of enzymes, the reactions would take place much too slowly to sustain life.
Define enzymes;
protein molecules, catalyse a specific reaction, unchanged by
the reactions they catalyse
Whats the name of the model - enzyme and substrate bind?
Lock and Key Model
name of site on enzyme where substrate binds to?
Active site
Lock and key model - step by step
1) Substrate binds to enzyme (in active site) reversibly
2) Non-covalent interactions (enzyme-substrate complex) > reaction takes place
3) enzyme-product complex
4) product leaves to make free enzyme and free substrate
What INDUCED fit mean vs lock and key model
the active site of an enzyme will undergo a conformational change when binding a substrate, to improve the fit.
Chemical and shape compatibility of a substrate with an enzyme active site (substrate fits to active site precisely)
in the active sites what are two things that are present?
> hydrophobic region
charges +/-
Enzymatic reaction Profile -
Enzyme works as catalyst providing an _______ pathway for the reaction in which the rate-determining step has a lower ______ activation energy than that of the non-catalysed reaction
alternative
Gibbs
LOWER the activation energyβ¦
more sufficient the reaction is = the FASTER the rate of reaction
Two distinct steps in enzyme catalysed reaction:
1) first step - reversible
2) second step - irreversible
Michaelis Menten model was developed to understand the kinetics of enzymic reactions.
E + S <> (k1/k-1)
ES > E + R (k2)
A number of assumptions are applied to the Michaelis Menten model:
The enzyme binds a single substrate (the model assumes cofactors are already bound)
There are two distinct steps. The substrate binds reversibly but product formation is irreversible
A steady state approximation applies (quantitative analysis of the kinetics of enzyme-catalysed reactions)
Whats the name of the state at the strart of an enzymatic reaction?
The steady state
Rate of formation of ES = Rate of consumption of ES (conversion to E + P)
( π [π¬πΊ])/π π=(Rate of formation of ES - Rate of consumption of ES)=0
=k1 [E] [S] -k2 [ES] - k-1 [ES]
( π [π¬πΊ])/π π=k1 [E] [S] β(k2 - k-1 )[ES]=0
Assumption is made that: [E]0 = [ES] + [E] where [E]0=
total conc of enzyme
Concentration of the substrate is usually much larger than that of the enzyme, so that [S] is effectively ______
constant
In the initial phase of the reaction: [E]^ and [ES]^
[ES] and [E] remain steady over most of the reaction time.
Only at the end of the reaction, when [S] is exhausted, does [ES] decline
The Michaelis Menten Equation
π½ (ππππ ππ ππππππππ)= ( π½πππ [πΊ])/(π²π΄+[πΊ])
What kind of linear when the velocity of an enzyme catalysed reaction increases
a non-linear fashion
KM=the Michaelis constant.
It is (k2 +k-1)/k1
Maximal Velocity Vmax, when is Vmas reached on graph?
At the highest point before the gradient goes flat
When independent of the [S] = the rate of reaction remains constant at the maximum value, Vmax, and the reaction is zero order with respect to [S].
When [S] is large, [S]»_space; KMβ¦
a) π½ (ππππ ππ ππππππππ)= ( π½πππ [πΊ])/([πΊ]) = Vmax =K2 [E]0
(S cancelles each other)
b) Rate of reaction is now independent of [S]
c) All of the enzyme molecules have S attached during the reaction and [ES] = [E]0
d) The enzyme is said to be saturated. If the [S] increases, no more enzymeβsubstrate complexes can be formed, and the rate of the reaction is independent of [S]
Vmax;
the maximum rate that a given enzyme can achieve, indicates the velocity of the reaction when the enzyme active site is saturated and indicates the maximum number of moles of S that can be processed in a unit of time (mol s-1)
How do we know it is first order?
The rate of reaction increases linearly with [S]
It is first order with respect to [S].
How do we know it is first order?
The rate of reaction increases linearly with [S]
It is first order with respect to [S].
Define the turnover number;
number of substrate molecules converted into product by an enzyme molecule in a unit time when the enzyme is fully saturated with substrate
What is the turnover number related to?
It is related to Vmax and is equivalent to the rate constant k2
Turnover number equation
Turnover Number = K2 = Vmax/[E]0 (s-1)
Range from 1 to 104 s-1 in MOST enzymes
Km, whats it used for?
The Michaelis constant = (k2 +k-1)/k1
gives indication of how tightly the enzyme binds its substrate
KM= substrate concentration ([S]) at which the reaction velocity is ____ of V maximum (Vmax/2)
half
Weak substrate; large or small Km?
large Km
a high [S] is needed to achieve Vmax/2 (and even more to reach Vmax!)
A good substrate has a ____ Km;
low
Only a low [S] needed to reach Vmax.
KM= The [S] at which the enzyme active site is ____ saturated
half
Lineweaver-Burk Equation
The y and x axis intercepts give us the important kinetic terms Vmax and KM from easily measured properties (rate and substrate concentration)
Why use a straight line graph than a curve?
easier to plot
