What are Enzymes?
- Enzyme: globular catalytic protein
- All enzymes are catalysts, but not all catalysts are enzymes
- Catalyst: chemical agent that speeds up a reaction without being consumed by the reaction → can be used over and over again
- Do not affect the free energy of a reaction!
Localization of Enzymes
- Some enzymes are grouped into complexes, membranes, & organelles (mitochondria) → increase the efficiency of metabolic process
Activation Energy Barrier
- Reacting molecules must collide and have enough energy (AE) needed to reach transition state and break bonds of reactants
- Activation energy often heat from environment
- But bad bcuz speeds up all reactions, denatures proteins & kills cells
- Transition State: reactive (unstable) condition of the substrate after enough energy has been absorbed to start the reaction
- Activation energy often heat from environment
How Enzymes Speed Up Reactions
- Stabilizes transition state and lowers Ea Barrier
- Allow chemical reactions to occur at lower temperatures
- Speeds up natural reactions ≠ cause them
- Reactions can occur without enzymes, but would be slower & a lot more energy
Substrate Specificity of Enzyme
- Substrate: reactant an enzyme binds/acts on
- Enzyme-substrate complex: enzyme binds to substrate and then substrate becomes product
- Enzymes might have more than one substrate but always only catalyze ONE type of reaction for each → gives enzyme specificity
- Specificity of an enzyme is bcuz of shape & polarity which results from amino acid sequence so molecules with (-) charge interact with (+) charged side chains in the active site
- Enzymes have an active site that binds to substrate because of polarity or shape.
Induced-Fit Model: describes how enzymes work
Enzyme and substrate not perfect fit → interactions between enzyme’s amino acids and substrate causes enzyme/active site to change shape and bind tighter to substrate → enhances enzymes ability to catalyze the reaction
Enzyme Cycle
- The enzyme binds to substrate forming enzyme-substrate complex
- Substrate held by weak bonds
- Active site lowers Ea barrier and speeds up reactions by…
- Orienting substrates correctly to collide (physically)
- Providing favorable microenvironment
- Straining substrate bonds (breaking bonds, hydrolysis)
- Covalently bonding to the substrate
4. Substates become products
Process Notes
- Enzymes are unchanged by reactions
- Enzyme catalyzes a reaction in both forward and reverse directions
- Direction of net activity is determined by substrate concentration
- Net direction of enzyme can be driven forward by keeping product of low (remove it or convert to another product)
Environmental Factors/Local Activity on Enzyme Activity
- Each enzyme has optimal pH and temp
- Single amino acid substitution mutation can alter optimal pH and temp
- Temp increases = enzyme activity increases
- But only until too high temp & then decreases
- At extreme environmental conditions, proteins become denatured and lose function
- Lose 3D shape as HB and peptide bonds break down (irreversible) → can’t bind to substrate
- Most enzymes have optimal pH at 7; digestive proteins (ie pepsin) become active only at low pH
- Saturated enzyme has all active sites engaged → cell might make more enzymes
- More substrate = increased activity until a certain point
Cofactors: nonprotein helpers for catalytic activity (ex. Zinc)
- Coenzyme: organic cofactors that binds to enzymes active site and increases catalytic activity, usually by donating or taking electrons (ex: vitamins)
- Inorganic Cofactors: often metal ions
- Regulate enzyme activity = regulate metabolism/reaction
Enzyme inhibitors
- Bind to enzyme and reduce function
- Some bind irreversibly while others held by weak ionic or HB
- Competitive inhibitor: mimics substrate and binds to active site which blocks substrates
- More substrate can overcome inhibition
- Noncompetitive inhibitor: binds to diff site on enzyme, irreversible
- Changes shape to make active site less effective
- Toxins, antibiotics, chemicals
- Changes shape to make active site less effective
Allosteric Regulation
- Regulatory molecules bind to allosteric sites and affect enzyme shape → function/efficiency
- Activators – lock enzyme to be active
- Inhibitors – lock enzyme to be inactive
- Cooperativity: binding of substrate increases catalytic activity at other sites (allosteric activation)
- Often occurs in enzymes that consist of multiple subunits (4th), each with own active site
- Ex: One substrate molecule primes an enzyme to act on additional substrate molecules more readily