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enzymes - substances that carry out most of the catalysis in living organisms  

  • RNA may also carry out some catalysis
  • substrates - molecules undergoing a reaction
  • temporarily stabilizes an association between substrates, lowering activation energy
  • carbonic anhydrase - increases production of carbonic acid from 200/hr to 600,000/sec
  • active sites - pockets/clefts on the enzyme where substrates bind to form enzyme-substrate complex
  • substrate must fit perfectly in an active site for catalysis to work; proteins adjust shapes into an induced fit between it and the substrate
  • amino acid side groups of enzymes stress/distort certain bonds, weakening them

enzymes that take many forms - some function as parts of cell membranes/organelles

  • multienzyme complex - associations of several enzymes catalyzing different steps in a reaction sequence
    • doesn't require that products of 1 reaction dissociate to move on to the next enzyme
    • no unwanted side reactions
    • all reactions within the complex can be controlled as a unit
  • pyruvate dehydrogenase - controls entry to Krebs cycle
  • fatty acid synthetase - catalyzes synthesis of fatty acids from 2-carbon precursors
  • RNA catalysts - aka ribozymes
    • intramolecular catalysis - catalyze reactions on themselves
    • intermolecular catalysis - act on other molecules; ribozymes don't change
    • adds more support for belief that RNA came before proteins

factors affecting enzyme activity  

  • temperature - increase in heat leads to increase in random molecular mov't
    • higher temperature adds stress to bonds
    • rate increases w/ temperature up until optimum temperature
    • proteins denature above the optimum temperature
  • pH - controls balance between positively/negatively charged amino acids
    • ionic interactions hold enzymes together
    • optimum pH - ranges from 6 to 8
    • ionic interactions dependent on hydrogen ion concentration
  • inhibitor - substance that binds to an enzyme and decreases its activity
    • feedback inhibition - end product of biochemical pathway acts as inhibitor of an earlier reaction on the pathway
    • competitive inhibitor - competes w/ substrates for same active site
    • noncompetitive inhibitor - binds to enzyme in a location other than the active site; changes the enzyme's shape so that the substrate won't fit
    • allosteric inhibitor - substance that binds to an allosteric site (chemical on/off switch) to reduce enzyme activity
  • activators - binds to allosteric sites and increase enzyme activity
  • enzyme cofactor - additional chemical components that assist enzyme function
    • coenzyme - nonprotein organic molecule
    • serves as an electron acceptor and transfers electrons to substrates in another reaction
  • nicotinamide adenine dinucleotide (NAD+) - made of NMP and AMP bonded together
    • AMP acts as core
    • becomes NADH when reduced, can now supply 2 electrons and a proton for other reactions
Subject X2: 

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