Enzymes Ch 2 Flashcards
Enzymes
Biological catalysts
Lower activation energy
Increase rate of reaction (kinetics)
Are not consumed (appear in both reactants and products)
PH and temp sensitive
Do not affect overall change in energy
Specific for particular reactions or class of reactions.
Catalyst
Do not change the thermodynamics of biological reaction (change in enthalpy reaction H and equilibrium position do not change)
Enzyme specificity
A certain enzyme will only catalyze a certain reaction’s or group of reactions’ substrates
Major enzyme classifications
Ligases Isomerases Lyases Hydrolases Oxidoreductases Transferases
Lil Hot
Oxidoreductases
Catalyze oxidation-reduction reactions
Often have cofactor that acts as an electron carrier such as NAD+ or NADP+
Electron donor is known as reductant
Electron acceptor is oxidant
Transferases
Catalyze movement of functional group from one molecule to another
Kinases are also a member of this class, catalyzing the transfer of a phosphate group generally from ATP to another molecule.
Hydrolases
Catalyze the breaking of a compound into two molecules using the addition of water
Example phosphatase which cleave phosphate from another molecule
Lyases
Catalyze the cleavage of a single molecule into two products; do not require water
Reverse is possible and often fulfilled by lyases then referred to as syntheses
Isomerases
Catalyze arrangement of bonds within a molecule
Some can also be classified as oxidoreductases, transferases, or lyases depending on the mechanism of the enzyme. Catalyze reactions between stereoisomers and constitutional isomers
Ligases
Catalyze addition of synthesis reactions generally between large similar molecules as often require ATP
Most likely encountered in nucleic acid synthesis and repair
Endergonic reaction
One requiring energy input (change in energy is greater than zero)
Exergonic reaction
One providing energy output (change in energy is less than zero)
Activation energy
Energy required for substrate to reach transition state
Enzyme- substrate complex
The physical interaction between a substrate and its active site on the enzyme.
Lock and Key Theory
Less widely accepted than induced fit model.
The enzyme’s active site is already in the correct conformation to bind to substrate and does not change
Induced fit model
More scientifically accepted theory of enzyme substrate interaction
- the enzyme’s active site conforms as the substrate approaches
Induced form or transition state is better for both molecules
Cofactors
Nonprotein molecules
Generally inorganic molecules or metal ions that are often ingested as dietary minerals. Small in size to find to active site of the enzyme and participate in catalysis, usually by charge through ionization, Protonation, or deprotonation
Coenzymes
Small. Low amounts in cells.
Participate in catalysis by carrying a charge through ionization, protonation, or deprotonation.
Generally small organic groups, vast majority are vitamins or vitamin derivatives such as NAD+, FAD and coenzyme A.
Water soluble: B complex and vitamin C
Fat-soluble: vitamins A, D,E, and K better regulated by partition coefficients
Apoenzymes
Enzymes without their cofactors
Holoenzymes
Enzymes with their cofactors
Prosthetic groups
Tightly bound cofactors or coenzymes that are necessary for enzyme function
V max
Maximum velocity
Enzymes are fully saturated with substrates and reaction rate is producing products as fast as possible.
Adding substrates after this point cannot increase rate, only adding enzymes
Michealis-Menton Plot of Enzyme kinetics
Relation between reaction velocity and substrate concentration [S]
Level off at v max
Michealis-Menton equation
Rate of reaction, v, depends on concentration of the enzyme [E] and substrate [S] which forms the product [P].
Enzyme-substrate complexes form at rate k1 and dissociate at rate k2 or turn into enzyme plus product at rate of k3
Equation for relation of enzymes to substrates where enzyme is held constant
v=(v max [S])/(K m + [S])
When reaction rate is equal to half of v max
K m=[S]
V max over 2 equals v max times concentration of S over Km plus concentration of S
So v max times (Km plus concentration of S) equals 2(v max times concentration of S)
So Km plus concentration of S equals 2 times concentration of S so Km equals S
Michealis constant Km
Understood to be the substrate concentration at which half of the enzyme’s active sites are full
Under some conditions it is the measure of the affinity of the enzyme for its substrate - higher Km means lower affinity for substrate
Cannot be changed by altering concentration of substrate or enzyme
Lineweaver-Burk Plots
Double reciprocal graph of michealis-menton equation
Intercept of x axis gives value of -1/Km
Intercept of y axis gives value of 1/v max
Allows to compare Km and v max wo estimation
Cooperativity
Multiple subunits and multiple active sites
Subunits and enzymes may exist in
Low affinity tense state T
High affinity relaxed state R
Binding of substrate encourages other subunits to change from T to R
Or loss of substrate results in opposite
Feedback regulation
Enzymes regulated by products further down the pathway
Feed forward regulation
Less common than feedback regulation; enzymes regulated by intermediates that precede the enzyme in the pathway.
Negative feedback
Feedback inhibition; helps maintain homeostasis. Once there is enough of the product it turns off the pathway that creates the product
Product may bind to active site of enzyme that acted earlier in its biosynthetic pathway (competitive inhibition)
Competitive inhibition
Reversible inhibition- higher Km and same v max.
Occupies the active site. Can be overcome by adding more substrate
Noncompetitive inhibitor
Reversible inhibitor
Binds to allosteric site instead of active site - induces change in conformation
Cannot be overcome by addition of substrate
Bind equally well to enzyme and enzyme-substrate complex
No change to Km; v max is decreased
Mixed inhibitors
Reversible inhibition
Can bind to enzyme or enzyme-substrate complex but does not have same affinity for each. Do not bind at active sites but allosteric instead
If enzyme binding is preferred Km is increased
If enzyme-substrate complex binding is preferred Km is decreased
V max is decreased
Uncompetitive inhibitors
Reversible
Bind only to enzyme-substrate complex and lock substrate in enzyme preventing its release
Increased affinity between enzyme and substrate
Lower Km and v max
Irreversible inhibition
Active site unavailable for a long time or permanently
Asa is a good example
Allosteric enzyme’s
Regulated enzyme Multiple binding sites one is active and others regulate availability of active site
May be active or inactive may be bound to by allosteric activators (activator shift to make active site more available) or allosteric inhibitors (opposite)
May alter activity of enzyme
Covalently modified enzymes
Enzymes may be activated or deactivated with phosphorylation or dephosphorylation, maybe either one
Zymogens
Enzymes that are dangerous if uncontrolled and will consume and organ. Contain active or catalytic domain and regulatory
