Lecture 22- introduction to enzymes and kinetics Flashcards
Enzyme overview
Protein catalysts
Increase rate of chemical reactions
Bind substrates (reactants)
Release products
Enzymes can be … during a reaction
Modified
But they return to their original for and aren’t used up
Enzymes can act as a point of ….
Regulation
Enzymes are key players in …. pathways
Metabolic
Enzyme substrate-binding sites
Substrates bind to specific sites via interactions with enzyme’s amino acids
Spatial geometry dictates specifcity
Enzyme active catalytic site
The region where the reaction occurs
Functional groups present include co-enzymes, metal ions and amino acid residues
Enzyme activation energy and transition states
Substrate is activated by functional groups
Energy needed to form high-energy intermediate reduces
Enzyme pH and temperature profiles
Enzymes have a functional pH & temp range
Increased temp -> increased reaction rate but can be denatured
Enzymes …. the position of equilibrium in a reaction
Do not alter
They accelerate the establishment of the position of equilibrium
Enzymes involved in …. reactions are more specific that those involved in …….
Biosynthetoc
Degredation
Lock and key model
Complementary 3D surface that recognises the substrate
Substrate binds through hydrophobic, electrostatic interactions and hydrogen bonds
Binding can be prevented by steric hinderance and charge repulsion
The induced-fit model
As substrates bind, enzymes undergo conformational change
Side chains of amino acids reposition
Binding interactions increase
Not a right lock but a dynamic surface
Enzyme active site
Cleft or crevice formed by polypeptide chain
3D arrangement permits reacting substances to approach each other
Enzyme transition state complex
Unstable high energy complex with strained electronic configuration
activation energy of formation is reduced compared ti the non-catalysed reaction
Transition state decomposes to products and enzyme returns to original form
What are cofactors/coenzymes?
Molecules that help an enzyme or protein function properly
What are tightly bound cofactors known as?
Prosthetic groups
What are coenzymes usually synthesised from in humans?
Vitamins
Multi-enzyme complexes
Formed to promote consecutive reactions in a metabolic pathway
eg. pyruvate dehydrogenase
Advantages: transit time via diffusion reduced
less interference
Isoenzymes (isozymes)
Enzymes that differ in amino acid sequence but catalyse the same chemical reaction
May show different kinetic parameters
eg. lactate dehydrogenase
Enzyme catalysed reaction equation
E + S ⇌ [ES] ⇌P + E
Factors affecting initial rate of enzyme reactions
[substrate]
[enzyme]
ph, temp, activators, inhibitors
Michaelis-Menten equation enzymes
v= (Max[S])/(Km+[S])
Michaelis-Menten equation meaning
Assumes: number of molecules and is large (so large enzyme number) & % of total enzyme-bound substrate is low
Equation relates initial velocity to concentration of substrate and two parameters Km and Vmax
Michaelis Menten: at …. concentrations ([S]»Km) all active sites are occulpied
high
Reaction rate is independent of [substrate]
No more enzyme substrate complexes can be formed
What is Km in the Michaelis Menten equation?
Michaelis constant
Substrate concentration at which the initial rate is 1/2Vmax
Michaelis Menten: at low concentrations ([S]«Km) active site occupancy is ….
Low
What is Vmax in Michaelis Menten equation?
Maximum rate achieved by the system
Low/high Km
Low Km= high substrate affinity
High Km= low substrate affinity
Catalytic efficiency (η)
η=kcat/Km
Maximum catalytic efficiency
Theoretical limit about 10⁹M⁻¹S⁻¹
Some enzymes have evolved maximum catalytic activity
Lineweaver-Burk
1/V= (Km/Vmax) x (1/[S]) + (1/Vmax)
Can be used to work out Km and Vmax
Equation is same form as y=mx+c
Enzyme units
1 unit of enzyme activity (U)= amount of enzyme which transforms substrate into product at an initial velocity of one micro mol per minute measured under defined assay conditions
