enzyme kinetics

catalysts

• speed up attainment of reaction equilib rium

• kinetics change

• thermodynamics do not

how much faster are catalysed reactions compared to non catalysed

10^3 - 10^ 17 faster

which form is in vivo glucose incorportated

beta form

how do enzymes lower the activation energy

• enzymes reduce the energy barrier by providing alternate reaction pathway with lower activation energy

when is active site most complementary

transition state

• the stabilisation lowers the energy barrier

examples of cofatcors

• essential ions

• coenzymes

  • cosubstrates

  • prosthetic groups

essential ions

loosely bound or tightly bound in metalloenzymes

• eg magnesium with ATP

prosthetic groups def and examples

• conzyme of cofactor bound tightly to enzyme

• eg haem, vitamins

cosubstrate

• coenzyme

• lossely bound and needs to be recycled

• eg ATP

rate equation

v = k[s]

• velocity is proportional to concentration of substrate

• initial velocity is directly proportional so 1st order for linear processes

which step is rate limiting

catalytic

curve and rate equation for catalysed reactions

• curve is hyperbolic

• linear at low conc of substrate

• at high conc, 0 order kinetics, full saturation, changing conc has no effect

  • due to active site of enzyme

michaelis menten equation

• Vmax is max velocity

• Km is the michaelis constant

• Vo is the initial velocity

  Vo = Vmax * [S] / Km + [S]

Km

• measure of affinity of the enzyme for the substrate

• enzyme-substrate disassociation constant

• the concentration of substrate needed for half of the maximum velocity

k2

• in the michaelis constant

• the rate of catalysis

• negligible more or less k-1/k+!

maximum velocity

• the velocity when the enzyme is saturated with substrate

• proportional to enzyme concentration

Kcat

• measure of the number of molecules of substrate converted to product per second per active site

equation for the maximum rate at saturating [S]

• Vmax = kcat[Et]

• [Et] is the total enzyme concentration - number of active sites

when does kcat = k2

in simple reactions

the higher the kcat / km

• the more efficient the reaction

which step is slow in michaelis menten equation

• formation of product is the slowest step k2

• so k2 is negligible

what is a rate constant at low [S] and use

• kcat/km

• compares catalytic efficiency of different enzymes

• relative abilities of different compounds to serve as a substrate for an enzyme

lineweaver burk plot

• linear transformation of the michaelis menten equation

• plot reciprocal 1/vo versus 1/[S]

what is the x axis intercept on a lineweaver burk plot

• -1/Km

what is the y axis intercept on a lineweaver burk plot

1/Vmax