Lecture 5: Introduction to Enzymes Flashcards
Collision Theory:
states that chemical reactions can occur when atoms, ions and molecules collide
Activation Energy
- energy needed to initiate a chemical reaction
- disrupts electronic configurations (to initiate the reaction)
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
What happens to the delta G in an enzymatic reaction
it DOES NOT change!
Enzymes alter the activation energy only
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
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
Enzyme specificity
- there is usually only one enzyme for one reaction
lots of enzymes!
Importance of cofactors
- enzymes are proteins but many need nonprotein components (inorganic or organic) for catalytic activity
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
apoenzyme
protein portion of an enzyme
inactove
cofactor
non-protein portion of an enzyme
activator
holoenzyme
whole enzyme
active
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)
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
- free energy change between the products and reactants
- energy required to initiate the conversion of reactants into products (activation energy, Ea)
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)
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
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]
What do different Keq values mean
- “large” or positive exponents –> products favored, exergonic
- “small” or negative exponents –> reactants favored, endergonic
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
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
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
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
Lock and key model
- NOT accurate
- fits perfectly together from start
induced fit model
- ACCURATE
- more like a handshake
- enzyme and substrate come together and alter shape a little to fit
