Lecture 5: Introduction to Enzymes Flashcards Preview

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Flashcards in Lecture 5: Introduction to Enzymes Deck (44):
1

Collision Theory:

 

states that chemical reactions can occur when atoms, ions and molecules collide

2

Activation Energy

 

- energy needed to initiate a chemical reaction

- disrupts electronic configurations (to initiate the reaction)

3

Reaction rates

 

- the frequencies of collisions with sufficient energy to make chemical reactions occur

- can be increased by enzymes or by supplying thermal energy or pressure

- enzymes lower the activation energy but DO NOTalter their free energy changes

4

What happens to the delta G in an enzymatic reaction

 

it DOES NOT change!

 

Enzymes alter the activation energy only

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5

Acceleration of chemical reaction by  enzyme

A reaction could take years to happen (or basically never seem to happen)

an enzyme makes it occur within minutes or seconds

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6

enzymes as biological catalysts

 

- each is specific for a biological reaction

- they act on specific molecules at the beginning of the reaction (substrates) and convert them into products

- turnover rate of 1 - 10,000 molecules/second --> but can be evern higher

7

Enzyme specificity

 

- there is usually only one enzyme for one reaction

lots of enzymes!

8

Importance of cofactors

 

- enzymes are proteins but many need nonprotein components (inorganic or organic) for catalytic activity

 

9

Definitiions of enzyme parts and cofactors

apoenzyme: protein part of the enzyme without cofactor

cofactor: prosthetic groups (tightly bound to an enzyme) or coenzymes (released from the enzymes active site during the reaction)

holoenzyme: apoenzyme + cofactor

10

apoenzyme

 

protein portion of an enzyme

inactove

11

cofactor

 

non-protein portion of an enzyme

activator

12

holoenzyme

 

whole enzyme

active

13

coenzyme

 

- small organiz molecules that transport chemical groups from one enzyme to another

- some are dietary precursors or vitamins (cannot be made in the body and must be acquired from diet, deficiency leads to disease)

- chemically altered as a consequence of enzyme action (usually due to group transfer)

14

Important thermodynamic properties of enzymatic reactions

 

* reaction is likely exergonic (- delta G), if it is endergonic (+ delta G) it mnust be coupled with an exergonic one

 

1. free energy change between the products and reactants

2. energy required to initiate the conversion of reactants into products (activation energy, Ea)

15

Free energy change, delta G

 

(+, -, 0 ?)

 

- reaction occurs spontaneously if its free energy change is negative

--> exergonic reaction = - delta G

--> endergonic - + delta G --> requires input of free energy

- system is at equilibrium if delta G = 0

 

* depends only on the free energy of the products (final state) and free energy of the reactants (initial state)

 

16

Delta G Equation for

 

A + B C + D

 

Delta G0= standard free energy change

(delta G when each of reactants and products is at 1M)

R = 8.314 j/(mol x k)

T = temp in kelvin

 

use each concentration of reactants and products

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17

delta G0 at equilibrium

 

K eq =

 

delta G = G0 + RT ln ([C][D]/[A][B])

0 = G0 + RT ln ([C][D]/[A][B])

G0 = - RT ln ([C][D]/[A][B])

- G0 / RT = ln ([C][D]/[A][B])

*10 to the power of each side

 

k eq= [C][D] / [A][B]

18

What do different Keq values mean

 

- "large" or positive exponents --> products favored, exergonic

- "small" or negative exponents --> reactants favored, endergonic

19

How do enzymes interact with equilibrium?

 

- they do not alter the equilibrium itself but alter its rate

- they lower the difference in free energy between the transition state and the substrate and thus facilitate the formation of the transition state

20

Enzyme effect on Ea and transition state?

- lower the energy required to get to the transition state

 

overall delta G of the reaction is UNCHANGED

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21

enzyme substrate complex

 

- when enzymes come together with a substrate and promote the formation of the transition state

- substrate binds to active site on enzyme

22

How do enzymes lower activation energies?

 

- orient substrates correctly

- strain the substrate bonds (more easily broken)

- provide favorable microenvironment

- bond the substrate

 

*physically modify substrate

23

Lock and key model

 

- NOT accurate

- fits perfectly together from start

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24

induced fit model

 

- ACCURATE

- more like a handshake

- enzyme and substrate come together and alter shape a little to fit

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25

General enzyme mechanism

 

1. substrate binds to active site

2. enzyme facilitates the reaction

3. product is released

4. enzyme is NOT permanently changed and is recycled

5. continues to react with other substrate molecules

 

 

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26

What "shape" does the enzyme fit?

 

- the transition state

- when substrate first comes into contact it does not fit well

- enzyme then adjusts it/forces it to fit --> puts it in transition state

- then it is easier to break etc

27

Classifications of enzymes

 

1. oxidoreductases

2. transferases

3. hydrolases

4. lysases

5. isomerases

6. ligases

28

oxidoreductases

ox/red reactions

29

transferases

 

transfer of functional groups

30

hydrolases

 

hydrolysis reactions

31

lysases

group elimination to form double bonds

32

isomerases

 

isomerization

33

ligases

 

bond formation couples with ATP hydrolysis

34

michaelis menten model (process of reaction)

E + S ES E + P

 

enzyme + substrate

Ensyme substrate complex

Enzyme + product

* first step can be reversed

* second unlikely to reverse

35

behavior of mechaelis menten curve

 

- substrate is added to an enzyme, the reaction rapidly achieves steady state in which the rate at which the ES forms balances the rate at which it breaks fdown (fast at first, then slows to the max velocity)

- as substrate concentration increases, the steady state activity of a fixed conc of enzyme increases in a hyperbolic fashion to approach Vmax (max velocity) at which all the enzyme has formed a complex with the substrate

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36

Km

substrate concentration that results in a reaction rate equal to one half of Vmax

- considered "when enzyme is wokring well"

- V0 and Vmax doesnt really show this (they are extremes)

- Vmax/2 is like the average between the two

37

michaelis menten equation

 

uses Km relate the initial velocity to the substrate conc [S] and Vmax

 

 

 

 

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38

Initial velocity and substrate conc

 

The greater the concentration the greater the initial velocity

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39

Vmax

 

Km is the [S] that yields a velocity of 1/2 Vmax

When [S] = Km then V0 = Vmax/2

 

Vmax is approached asymtotically

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40

Lineweaver burke plot

 

- double reciprocal presentation of the michaelis mentedn equation

- y intercept is 1/Vmax

- x intercept is -1/Km

- slope = Km/Vmax

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41

Lineweaver buke equation

 

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42

Catalytic efficiency

 

Kcat = equivalent to the number of substrate molecules converted to product in a given unit of time on a single enzyme molecule when it is saturated with substrate

 

Kcat = Vmax /[ET}

 

ET = total enzyme concentration

 

* looks at saturated conditions

43

Catalytic efficiency equation

 

V0 = [ET] [S] Kcat /Km

 

when [S] is << Km the concentration of free enzyme is nearly equal to ET

 

Kcat/Km are limited  by the rate E and S can diffuse togeher in an aq solution

44

Catalytic perfection

 

- Kcat/Km  is around 108 or 109

- rate at which Enzyme and substrate can diffuse together in aqueaous solution

- about as fast as the two could come together