enzymes Flashcards
what are co-factors?
small molecules that are not part of the enzyme, but are required for activity
what is an enzyme lacking a co-factor called?
apoenzyme
what is the complex of enzyme and co-factor called?
holoenzyme
examples of co-factors?
inorganic co-factors: metal ions, zn2+, organic co-factors: vitamin c for collagen
do enzymes alter position of equilibrium?
no, they increase rates of forward and backward reactions
what is kcat?
the number of molecules of S that one active site can convert into product per second
how is complementarity between active site and S achieved?
high specificity, enzymes can differentiate between optical isomers, positional isomers and presence/ absence of functional groups
who proposed the ‘lock and key’ model?
emil fischer, 1890s
who proposed the ‘induced fit’ model?
daniel koshland, 1954
what did daniel koshland propose?
that S causes change in 3D relationship of active site and these changes bring catalytic groups into correct orientation for reaction
when was structural evidence for induced model produced?
1970s
how does a reaction of S to P proceed?
through a transition state, which is neither the substrate or the product
Properties of the transition state (X‡) in catalytic reactions?
requires a high Ea (ΔG‡) to achieve X‡, unstable high-energy combination of reactants, can either form products or fall apart to reactants
how do enzymes lower the ΔG‡ for X‡ to form?
non-covalent interactions (van der waals, H bonds, etc.) form between S and active site when ESC formed, releasing e and lowers ΔG‡, forming X‡.
how is Ea lowered for reactions with chymotrypsin?
aspartic acid, histidine, serine brought together during tertiary structure formation, lowering Ea
function of chymotrypsin?
protease, uses a wide range of proteins as S (non-specific)
what is covalent catalysis?
active site contains reactive group, which temporarily is attached to S
example of covalent catalysis?
reactive Ser on chymotrypsin attaches to substrate
what is acid-base catalysis?
molecule other than water acts as a proton acceptor or donor
example of acid-base catalysis?
chymotrypsin uses reactive His as base catalyst, which deprotonates + enhances reactive Ser
how does the catalytic amino acid triad of chymotrypsin activate the active site?
Asp H bonds with His, holding it in place, His H bonds with Ser, activating it
reaction mechanism steps of chymotrypsin?
S docks onto active site, induced fit, catalytic triad brought into close proximity to S, enzyme folds around S, target peptide bond is oriented close to catalytic Ser, attacks bond, acyl-enzyme X‡ resolved by Nu attack by H2O, releasing N-terminal of S, regenerating enzyme
what is enzyme kinetics?
study of enzyme-catalysed reaction rates
Equilibrium of enzyme reaction?
E + S ⇌ ES ⇌ E + P
What does V = k1[S][E] mean?
rate of reaction is proportional both to the concentration of substrate and enzyme
E + S -> (k1 above ar.) ES
rate of formation of ES depends on the [E] and [S], and is governed by the rate constant (k1)
E+S⇌ k1ES⇌k2 E+P
k-1 for ES to E + S
k-2 for E + P to ES
rate constants showing the equilibrium between forming and degrading ESC
When was Michaelis-Menten equation produced?
1913
What is the Michaelis- Menten equation?
Vo = (vmax*[S])/(KM+[S])
What is Vo?
the initial rate of reaction
What is Km?
[S] that gives 1/2 maximum reaction rate - half of active sites are occupies by S
