2-7 Enzymes as Biological Catalysts Flashcards Preview

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Flashcards in 2-7 Enzymes as Biological Catalysts Deck (12):

What are the general characteristics of enzymes?

  • Carry out almost all chemistry required by living systems
  • Permit a wide range of chemical reactions in a narrow set of conditions
  • Allow rapid, efficient adjustments to environmental conditions
  • Enhance reaction rate tremendously
  • Require extreme specificity
  • Capable of being regulated


What are the common features of active sites on enzymes?

  • Occupy a small part of total volume of most enzymes
  • 3-D structure
  • Bind substrates through multiple weak, non-covalent interactions (electrostatic, H bonds, van der Waals, hydrophobic effect)
  • Water is excluded unless it is a reagent (active sites are in clefts of proteins)
  • Highly specific binding of substrate
  • Can include non-protein prosthetic groups and cofactors


What is the model for an enzymatic reaction?

(Formation of the enzyme-substrate complex involves induced fit)


What is the Michaelis-Menten equation?


What does Km mean in the context of the Michaelis-Menten reaction?

  • Km = [S] at which v = 1/2 Vmax
  • Km = ((k2 + k3) / k1)
  • The catalytic rate of an enzyme is most sensitive to [S] when [S] << Km
  • Knowing Km allows you to calculate what fraction of enzyme molecules are bound to substrate at any [S]:
    fES = (v / Vmax) = ([S] / ([S] + Km))
  • When k2 >> k3 (i.e., the substrate binds and dissociates many times before it goes on to form product), Km ≈ Kd (the dissociation constant for the ES complex)
  •     k2 >> k3 is relatively common; under these conditions, Km gives you an idea of the affinity of enzyme for substrate: high Km = low affinity, low Km = high affinity


What does Vmax mean in the context of the Michaelis-Menten equation?

Vmax = maximal velocity achievable for a specific concentration of enzyme

Although increasing [S] does not give further increases in initial velocity at Vmax, increasing [E] will increase the Vmax.  (At Vmax, enzyme binding sites are saturated with substrate, so increasing [E] can relieve that saturation.) 


What does kcat mean in the context of the Michaelis-Menten equation?

AKA k3, or the “turnover number” because it describes the number of substrate molecules that can be “turned over” (converted to product) by a single enzyme molecule in a given period of time

kcat  =  Vmax /  [E]t
(where [E]t =  total enzyme concentration)


How can the Michaelis-Menten equation constants be used to provide a comparison of enzyme efficiencies?


A higher kcat/Km implies higher efficiency, because kcat determines how quickly the ES compex is used and Km describes how much ES complex is available.


What is a Lineweaver-Burk plot?

A linearized form of the Michaelis-Menten equation. Can be useful with enzyme inhibitors.


What are the different types of enzyme inhibition?

  • Irreversible: bind very tightly to an enzyme and inactivate an essential functional group
  • Reversible: bind reversibly to an enzyme and temporarily inactivate it while bound
  1. Competitive: compete with substrate for binding to the active site; can be overcome by increasing [S]; Km appears to increase, but Vmax does not change (on L-B plot, same y-intercept but different x-intercepts)
  2. Non-competitive: bind to a site on the enzyme OTHER than the active site; CANNOT be overcome by increasing [S]; Vmax appears to increase, but Km does not change (on L-B plot, same x-intercept, but different y-intercepts)
  3. Uncompetitive


What does Ki mean with regard to enzyme inhibition?

Ki is the dissociation constant for the enzyme-inhibitor complex. It describes the strength with which an inhibitor binds to an enzyme.

  Higher Ki implies weaker inhibition. 


How can Lineweaver-Burk plots be used to calculate Ki?

  • (slope in the presence of inhibitor / slope in the absence of inhibitor) = α
  • α = 1 + ( [I] / Ki )
  • Solve for Ki