Enzymes

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

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

1. The enzyme binds to substrate forming enzyme-substrate complex                           
2. Substrate held by weak bonds
3. 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

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