Define enzymes
• Enzymes - globular proteins of high molecular weight that act as catalysts (RNA is exception)
• Enzymes are specific - most cellular reactions catalysed by enzymes
Faster reaction rate
What is the nomenclature for enzymes?
Named by adding -ase to end of substrate it catalyses , e.g. lactase, or to catalysed reaction e.g. polymerase
What do some protein enzymes require for their activity?
• Some protein enzymes require non-protein group for their activity
• Haloenzymes - contains non-protein group = co-factor (non protein group), apoenzyme (protein)
Isoenzymes - occurs in different molecular form but catalyse same reaction
Give an example of basic enzyme classification
e.g. oxidoreductases = oxidation-reduction reactions,
isomerases - isomerization reactions
How do enzymes work?
• Lowers EA by forming enzyme-substrate complex
- Free energy change and equilibrium constant unaffected by enzymes
What are the four steps in a catalytic reaction?
- enzyme and substrate are available, 2. substrate binds to enzyme,
- substrate converted to products,
- products are released
Describe the problems with the lock and key model
- Assume enzymes are rigid - they are flexible molecules
- Over and undersized molecules are observed to bond but not react
- Some reactions occur in fixed sequence
- Mechanism for reaction rate enhancement not clear
Describe the induced fit model
- Enzyme partially flexible, substrate influence enzyme final shape
- Some compounds bind to enzyme but not react as enzyme distorted too much
- Some molecules too small to induce proper alignment, therefore cant react
- Only right substrate capable of inducing proper alignment of active site
What is the Michaelis-Mentin equation?
v=(v_max [S])/(K_M+[S])
Describe the saturation kinetics
- reversible step: enzyme substrate(ES) complex formed
- Dissociation step: ES complex dissociates to give product and free enzyme
What assumptions are made in the MM equation?
Assumptions - high substrate conc. so enzyme saturated, ES complex established rapidly, negligible reverse reaction (2nd step)
Describe the Rapid equilibrium approach
Occurs in first step, product formation step slower than first step= ES complex always at equilibrium conc.
Describe the Quasi-steady state approach -
Initial substrate conc. > enzyme conc.
What is an enzyme inhibitor?
Compounds other than substrate that may bind to enzymes and reduce their activity
Can be either reversible or irreversible - irreversible form stable complex, reversible easily dissociate
Describe the three major classes on inhibitor mechanisms
- Competitive - inhibitor usually substrate analog, compete with substrate for enzyme active site => Vmax same, Km increases
- Non-competitive - not substrate analogs, bind on sites other than active sites and reduce enzyme affinity => Vmax decreases, Km same
- Un-competitive - inhibitors bind only to ES complex, no affinity towards free enzyme => Vmax decrease, Km decrease
