Enzyme Kinetics Flashcards
(19 cards)
What are enzymes?
- enzymes are proteins
- 3D structure provides an active site for substrate binding and catalytic conversion to products
- may be single proteins or may contain 2 or more subunits
What are catalysts?
Speed up attainment of reaction equilibrium but do not affect the equilibrium itself and the enzyme itself is unchanged
Rate of enzyme reactions
10^3 to 10^17 when catalysed
Properties of enzymes
Lower the activation energy of a reaction
Provides a different reaction pathway with lower ∆G
Describe substrate binding
- specific but not complementary
- active site fits better to substrate after binding as its conformation changes slightly = induced fit
- active site is most complementary in shape to (has highest binding energy for) the transition state = transition state stabilisation (lowers energy barrier)
- reaction of substrate is favoured by a proximity effect = more product formed
Role of enzyme active site residues
- binding energy
- catalytic functional groups e.g. proton donors/acceptors, metal cofactor coordination and groups that can form covalent bonds to the substrate
Types of cofactors
- Essential ions = loosely bound (acts as activator) or tightly bound in metalloenzymes
- Coenzymes = transient carriers of atoms (e.g. hydrogen) or functional groups (e.g. phosphate)
- Cosubstrates = loosely bound and need to be recycled by a different enzyme (e.g. ATP, SAM, CoA)
- Prosthetic groups = coenzyme or cofactor bound tightly to enzyme (e.g. haem, FMN)
Plot of enzyme-catalysed reactions
- Curve is hyperbolic
- Linear at low [S] = first order
- Independent of [S] at high [S] = zero order (reaches maximum velocity)
Michealis-Menten equation
Vmax x [S]
—————
Km + [S]
Where Vmax is the maximum velocity and Km is the michealis constant
What does Km represent?
- the enzyme substrate dissociation constant
- a measure of affinity of the enzyme for the substrate
- the lower the value of Km, the tighter the substrate binding
- Km is equal to the concentration of substrate needed for half maximum velocity
What does Vmax represent?
- the velocity when the enzyme is fully saturated with substrate
- Vmax is proportional to enzyme concentration
What is Kcat?
The catalytic constant
A measure of the number of molecules of substrate converted to product per second per active site
Kcat = Vmax/[Et]
Et is the total enzyme concentration
6 classes of enzymes
- Oxidoreductases
- Transferases
- Hydrolases
- Lyases
- Isomerases
- Ligases
What are oxidoreductases?
Dehydrogenases
Catalyse oxidation-reduction reactions
What are transferases?
Catalyse group transfer reactions
Usually via a covalent intermediate of the substrate with the enzyme/co-enzyme
What are hydrolases?
- largest class of enzymes
- catalyse hydrolysis reactions
- cleave a bond with the addition of water to the products
What are lyases?
- catalyse lysis
- cleavage of a C-C, C-O, C-N or other bond
- creates a double bond
- water is not added to products
- for some, the reverse reaction (addition to the double bond) is more important = called synthases
What are isomerases?
- catalyse isomerism reactions = structural change within a single molecule
- many different forms exist depending on the type of isomerism
- isomerism = the transfer of groups within molecules to yield different structural or geometric isomers
- molecular formula does not change and there is no loss of groups/atoms
What are ligases?
- smallest family of enzymes
- also called synthetases
- catalyse ligation of two substrates forming a covalent bond
- require chemical energy e.g. ATP hydrolysis
