Chemical Kinetics: Rate Laws ORDER OF REACTION rate (= −d[A] /dt) = k[A] x [B] y Overall order of reaction = x + y Example: rate = k[A] 2 [B] The reaction is second order in A first order in B overall reaction order = 1+ 2= 3 (sum of the exponents) DETERMINING THE RATE LAW Method of initial rates : Initially, we know [A] and [B] (and [C] = [D] = 0) initial rate = k 1 [A] o x [B] o y “o” ⇒ “initial” (t = 0) Vary [A] 0 and [B] 0 , measure initial rates
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Chemical Kinetics: Rate Laws
ORDER OF REACTIONrate (= −d[A] /dt) = k[A]x[B]y
Overall order of reaction = x + y
Example: rate = k[A]2[B]
The reaction issecond order in Afirst order in Boverall reaction order = 1+ 2= 3
(sum of the exponents)
DETERMINING THE RATE LAW
Method of initial rates:
Initially, we know [A] and [B] (and [C] = [D] = 0)
initial rate = k1[A]ox[B]o
y “o” ⇒ “initial” (t = 0)
Vary [A]0 and [B]0, measure initial rates
Sample problem
Given the following data for the reactionA + B → Z
What is the ratio of kcat/kuncat at 37°C?(body temperature)
Assume Acat = Auncat(Q: is this a good assumption?)
CATALYZED VS UNCATALYZED
-(28 - 72 kJ/mol)ln
kcat
kuncat
=(.0083 kJ/mol K)(310 K)
= 17.1
>kcat
kuncat
= 3 x 107
speeds up by a factor of 30 million!
Peptidase enzymes – break up proteins into amino acids(in your stomach)
similar effect on Ea
Without these it would take ~ 300 years to digest a steak!
kcat
kuncat= e -(Ea,cat - Ea,uncat)/RT
ENZYMESEnzymes are biological catalysts.
Enzymes are produced by organisms to accelerate and tocontrol reaction rates.
Enzymes are typically large protein molecules orcombinations of proteins with other molecules. Theregion where the substrate/s (reactant/s) bind is called theactive site.
Enzymes differ from man-made catalysts:More efficient.More specific.Rate can be controlled by changing enzyme activity.
ENZYME CATALYSIS
enzyme
binding
sites
reactant molecules
enzyme-substrate
complex
products
k = A e-E /RTa
1. Enzyme active sitesare ideally suited fortransition statebinding (lowers Ea)
2. Juxtaposition ofreactants ⇒high effectiveconcentration(increases A)
Nature’s catalysts – big organic moleculesspecifically designed for certain reactions.Rate acceleration by > 1010 (how?)
Each factor enhances rate by ≥ 105
CONTROL OF ENZYMESSome enzymes wait in the “off” state, such as blood-
clotting and digestive proteinsThey are activated (reacted to make the active form)when needed.
The active site depends on the enzyme conformation(shape):
• Metal ions are held in place by different sections ofthe protein sitting in close proximity.
• If this shape is altered, the active site no longerfunctions and the enzyme is “turned off.”
• Molecular shape depends on pH, temperature, andreactions of the enzyme.
ENZYME ACTIVE SITES
Denatured enzyme –parts of the active siteare no longer in closeproximity.
Representation of anactive site in anenzyme.
Competitive Inhibition: Another way to inhibit an enzyme is tobind a molecule to its active site, blocking any catalytic activity.
Many drugs and poisons work by this mechanism.
DRUGSPenicillin (antibiotic) blocks an enzyme that bacteria use to build cell
walls.People do not have this enzymeBacterial cells only are poisoned.
HIV-protease inhibitors bind to the active site of an enzyme that releasesthe viral coat proteins, preventing the production of the HIV virus.
Active site
HIV protease
Ritonavir (inhibitor)
ENZYMESMetal ions are often bound at the active site and serve as thereaction center of the enzyme.
The enzyme carbonic anhydrase uses a Zn2+ ion at its activesite to accelerate the reaction:
CO2 + H2O → H2CO3
In red blood cells, CO2 is converted to H2CO3 whichdeprotonates to form HCO3
-.HCO3
- leaves the cell and serves as a buffer for blood plasma.
In the lungs, HCO3- is re-protonated to form H2CO3.
Carbonic anhydrase converts H2CO3 back to CO2(g) and H2O.
Exhale!
VITAMINS
Vitamins are non-protein parts of enzymes, calledco-enzymes.
When combined with the protein part they makeenzymes.
Enzymes derived from vitamins play critical rolesin redox chemistry in the body, which is thesource of heat and energy.