L9 Flashcards
the 3D structure of enzymes provides an active site for (2):
- binding of substrate(s)
- catalytic conversion to product (s)
what do catalysts do?
speed up the attainment of reaction equilibrium - but do not affect equilibrium itself
Properties of enzymes (substrates, stereospecific, reaction specificity)
substrates: highly specific reactants for enzymes
stereospecific: enzymes usually act upon only one stereoisomer of a substrate
reaction specificity: enzyme product yields are essentially 100% (no formation of wasteful by-products)
What do enzymes do to the activation energy of a reaction?
Lower it
What is ‘induced fit’?
the active site fits better to the substrate after bding
The active site is most complementary in shape to (and has highest binding energy for) what?
the transition state = transition state stabilization (lowers energy barrier)
What is the proximity effect?
reaction of substrate(s) is favoured by proximity effect (reaction on enzyme increases the effective concentration of reacting groups)
what is the role of enzyme active site residues? (2)
- binding energy (non-covalent interaction)
- catalytic functional groups
What are cofactors?
- Essential ions
- coenzymes (cosubstrates + prosthetic groups)
What do kinetic experiments examine?
the concentration of product ( P ) formed (or substrate consumed (S)) per unit of time
what is the rate equation
delta (P) / delta (t) = v (velocity or rate) = k[S] (rate constant)
What does the Vo [S] plot usually look like
Linear (for chemical process) - 1st order: Vo = k[S]
For enzyme catalyzed reactions, what does the curve look like?
hyperbolic - linear at low [S], independent of [S] at high [S] (reaches maximum velocity)
What do the constants mean in the Michaelis-Menten equation? Km
Km ~ the enzyme-substrate dissociation constant
- measure of the affinity of the enzyme for the substrate (the lower the value of Km, the tighter the substrate binding)
- equals the concentration of substrate needed for 1/2 max velocity
What do the constants mean in the Michaelis-Menten equation? Vmax
Maximum initial velocity - velocity when an enzyme is saturated with substrate (high [S])
(Vmax is proportional to enzyme concentration)
What is Vmax equation?
Vmax = kcat[Et]
where Kcat is the rate constant for ES -> E + P (only process occuring since enzyme saturated)
and Et is the total enzyme concentration (i.e. no. of active sites)
What is kcat?
termed the catalytic constant / turnover number
kcat = Vmax/[Et] - a measure of the number of molecules of substrate converted to product per second per active side
What are the six classes of enzymes?
- oxidoreductases (dehydrogenases)
- transferases
- hydrolazes
- lyazes
- isomerases
- ligases (synthetases)
What do oxidoreductases do?
catalyze redox rxns
What do transferases do?
catalyze group transfer rxns
What do hydrolases do?
catalyze hydrolysis rxs (cleave a bond (i.e. C-O, C-N), phosphoanhydride with addition of water to the products
What do lysases do?
catalyze lysis: cleavage of a C-C, C-O, C-N, or other bond - leaving a double bond (distinct from hydrolase as water is not added to products)
What do isomerases do?
catalyze isomerism rxns (many dif forms exist - depending on type of isomerism)
isomerism: the transfer of groups within molecules to yield different structural or geometric isomers (molecular formula doesn’t change - no loss of groups/atoms)
What do ligases do?
catalyze ligation (joining) of two substrates (forming new C-C, C-O, C-N, C-S bonds) - requires chemical energy (ATP hydrolysis)
