Ch 8.4 Text

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  1. ·         Study of chemical reactions: __
    ·         Study of the rates of enzyme-catalyzed reactions: __
    • kinetics
    • enzyme kinetics
  2. ·         Consider AàP
    o   Rate (V) is the qu__
    §  Rate constant=__
    · V=k[A] (units are s-1)
    o   Reactions that are directly proportional to the reactant concentration are __

    o   __ and the corresponding rate equations are V=k[A]2 and V=k[A][B]

    §  Units are M-1s-1
    antity of A that disappears in a unit of time and equal to rate of appearance of P


    first-order reactions

    Bimolecular reactions
  3. §  If a reaction appears to be first order, but has two reactants (B is excess and A is low), the reaction is __
    §  __ occurs when the rate is independent of reactant concentrations 
    • pseudo-first-order reactions
    • Zero order
  4. ·         Investigating a reaction rate can be done by following the __ as a __. The amount of product formed __ with time, although eventually a time is reached when there is __.
    o   The enzyme is still converting S to P and P to S, but the reaction equilibrium is attained 
    • increase in reaction product as a fucniton of time
    • increases
    • no net change in the concentration of S or P
  5. ·         Easier to consider the __ and define the __as the number of moles of product formed per second when the reaction is just beginning (t=0). The amount of enzyme present is __
    o   When plotted, the rate of catalysis rises __as substrate concentration increases and then begins to __ and approach a maximum at higher substrate concentrations
    • forward reaction
    • rate of catalysis V0 
    • constant
    • linearly 
    • level off
  6. o   A model for this is: __
    o   Thus, V0 is determined for each substrate concentration by measuring the __.
    o   We want an expression that relates the __ to the __ and __ and the __. Our starting point is that the catalytic rate is equal tot eh __
    §  V0=k2[ES]
    · Rate of formation of ES= __
    · Rate of formation of EP= __
    §  Rearranging gives [E][S]/[ES]=(k-1+k2)/k1
    E+S ßà ES (the intermediate) ßà E+P

    rate of product formation at early times before P accumulates

    rate of catalysis to the concentrations of substrate and enzyme and the rates of the individual steps

    product of the concentration of the ES complex and k2

    • k1[E][S]
    • (k-1+k2)[ES]
  7. o   The Michaelis constant: __; it is __of enzyme and substrate concentrations. We get a new equation for [ES]à [ES]=([E][S]/KM)
    §  Because suvstrate is usually present at a much __concentration than that of the enzyme, the concentration of __is nearly equal to the __. The concentration of __ is equal to the __:
    · [E]=[E]T-[ES]
    · Subbing it in for E gives: [ES]=([E]TS]/KM)/(1+[S]/KM)
    • KM=(k-1 + k2)/k1
    • independent 
    • higher 
    • uncombined substrate
    • total substrate concentration
    • uncombined enzyme
    • total enzyme concentration [E]T minus the concentration fot eh ES complex
  8. §  More substitution gives the __, which is attained when the __—that is, when [ES]=[E]T. Thus: Vmax= k2[E]T. More substitution into another equation gives __
    ·       At very low substrate concentration, when [S] is much less than __, V0= __; that is, the reaction is __ with the rate directly proportional to the __. At very high substrate concentration, when [S] is much greater than __, V0=__; that is, the rate is __. The reaction is __, independent of substrate concentration. 
    • maxima rate, Vmax
    • catalytic sites on the enzyme are saturated with substrate
    • V0=Vmax ([S]/([S]+KM)
    • KM
    • (Vmax/KM)[S]
    • first order
    • substrate concentration
    • KM
    • Vmax
    • maximal
    • zero order
  9. §  KM is equal to the substrate concentration at which the reaction rate is __. KM is an important characteristic of an __ and is significant for its biological function
    • half its maximal values
    • enzyme-catalyzed reaction
  10. §  KM and Vmax values can be determined by several means
    · KM is equal to the __; but its never attained, only approached
    o   The Michaelis constant, KM, and the maximal rate, Vmax, can be readily derived from __ measured at a __
    A curve-fitting program can be used
    • substrate concentration that yields Vmax/2
    • rates of catalysis
    • variety of substrate concentrations if an enzyme operates
  11. · Besides a program, it can be derived algebraically by remembering that Vmax is approached __, meaning it is impossible to obtain a definitive value from a Michaelis-Menten curve. Because KM is the __, it is likewise impossible to determine an accurate value of KM. However, Vmax can be accurately determined if the Michaelis-Menten equation is transformed into one that gives a straight – line plot, called a __
    • asymptotically
    • concentration of substrate at Vmax/2
    • Lineweaver-Burk (or double-reciprocal) plot
  12. §  KM and Vmax values are important enzyme characteristics
    · For most, KM is around 10^-1 and 10^-7; but, it depends on thhe __ and __
    · The Michaelis constant (KM) has two meanings:
    o   __

    particular substrate and environmental conditions

    It is the concentration of substrate at which half the active sites are filled (provides a measure of substrate concentration required for significant catalysis to take place)

    o   It is related to the rate constants of the individual steps in the catalytic schemes of eq. 12 (in textbook)

  13. §  In the equation KM= (k-1+k2)/k1, if k-1 is greater than k2, what happens?
    ES complex dissociates to E and S much more rapidly than product is formed
  14. ·         KM and V­amx also permit the __. This relation of fES to KM and Vmax is given by
    o   FES= (V/Vmax)=([S]/([S] + KM))
    ·         Kcat/KM is a measure of __
    o   When the substrate is much greater than K, the rate of catalysis is __, which is a function of __
    §  Because most enzymes are not normally saturated with substrate, under physiological conditions, the [S]/KM ratio is between __
    • determination of fES, the fraction of active sites filled.
    • catalytic efficiency
    • equal to Vmax
    • kcat, the turnover number
    • 0.01 and 1.0.
  15. ·         When [S] << K, the enzymatic rate is __ because __
    o   The kinetics of an enzyme under these more typical conditions is:
    §  V0= (kcat/KM)[S][E]
    ·         When [S] << K, the __ is nearly equal to the __; so:
    o   V0= (kcat/KM)[S][E]T
    • much less than kcat
    • most of the active sites are unoccupied
    • concentration of free enzyme [E],
    • total concentration of enzyme [E]T
  16. ·         Thus, when [S] << KM­, the __depends on the values of kcat/KM, [S], and [E]T
    o   Under these conditions, kcat/KM is __
    §  The rate constant kcat/KM is a measure of __ because it takes into account both the __ with a particular substrate (kcat) and the __
    enzymatic velocity

    the rate constant for the interaction of S and E

    catalytic efficiency

    rate of catalysis

    strength of the enzyme-substrate interaction (KM)
  17. ·         Chymotrypsin clearly has a preference for __.
    cleaving next to bulky, hydrophobic side chains.
  18. ·         How efficient can an enzyme be?
    o   First: __
    §  KcatKM ratio depends on __
    ·         Suppose:
    o   The rate of formation of product (kcat) is __ than the rate of dissociation of the ES complex (k-1)à value of __ approaches k1. Thus, the ultimate limit on the value of kcat/KM is set by __

    §  This rate cannot be faster than the __ of an enzyme and its substrate.
    • determine whether there are any physical limits on the value of kcat/KM
    • k1, k-1, and kcat
    • much faster
    • kcat/KM
    • k1, the rate of formation of the ES complex
    • diffusion-controlled encounter
  19. ·         __ limits value of k1
    §  The kcat/KM ratios of the enzymes __, __, and __ are between 108 and 109 s-1M-1
    ·         Enzymes that have kcat/KM ratios at the upper limits have attained __. Their catalytic velocity is restricted only by the __.
    • Diffusion
    • superoxide dismutase, acetylcholinesterase, and triose phosphate isomerase
    • kinetic perfection
    • rate at which they encounter substrate in the solution
  20. o   Any further gain in catalytic rate can only come by __
    §  When enzyme and substrate come together, there may be __ on the enzyme that entice the substrate tot eh active site, called __
    • decreasing the time for diffusion of the substrate into the enzyme’s immediate environment
    • attractive electrostatic forces
    • Circe effects
  21. ·         The diffusion of a substrate throughout a solution can be overcome by __. Some series of enzymes are organized into complexes so that the product of one enzyme is __. Products are channeled from one enzyme to the next
    • confining substrates and products in the limited volume of a multienzyme complex
    • very rapidly found by the next
  22. §  Many A+ Bà P + Q reactions transfer a __ from one substrate to the other; others, like redox, transfer electrons.
    §  Multiple substrate reactions can be divided into two classes: __ and __
    • functional group
    • sequential and double-displacement
  23. ·         Sequential Reactions: __
    o   In a bisubstrate reaction, a __ of the enzyme and both substrates forms
    o   Are of two types: __ and __
    • all substrates must bind to enzyme before product is released
    • ternary complex
    • ordered (substrates bind in defined sequence) and random
  24. §  Ordered: __

    §  Random: __
    the coenzyme binds first and product is released first; the enzyme ezists as ternary complex consisting of, first, the enzyme and substrates and, after catalysis, the enzymes and products

    the order of the addition of substrates and release of products is random
  25. ·         Double-displacement (ping pong) reactions: __
    o   Has a __, in which the enzyme is temporarily modified
    o   Reactions that __ are examples
    • one or more products are released before all substrates bind the enzyme
    • substituted enzyme intermediate
    • shuttle amino groups between amino acids and alpha-ketones
  26. ·         __ do not obey michealis-Menten kinetics
    o   Michaelis-Menten model cannot account for __, especially those of __, which consist of multiple subunits and multiple active sites
    §  Allosteric enzymes have __
    • Allosteric enzymes
    • kinetic properties
    • allosteric enzymes
    • sigmoid curves
  27. o   In alloteric enzymes, the binding of substrate to one active site can __--> bidnding of substrate becomes __
    o   Activiry of an allosteric enzyme may also be altered by __ that are reversibly bound to specific sites other than the catalytic sites
    • alter the properties of other active sites in the same enzyme molecule
    • cooperative
    • regulatory molecules
Card Set:
Ch 8.4 Text
2014-10-06 01:40:08
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