Chapter 6 Flashcards
Enzymes
- are proteins
- act as a protein catalyst for biochemical reactions (have catalytic properties)
- accelerate the rate of a reaction
- decrease activation energy for a reaction to proceed
- enzymes will not change the equilibrium of a reaction
- highly specific for substrates
- will facilitate conversion of a substrate (reactants) into products
- are not degraded or destroyed during a reaction
- activity is measured in umol/min/mg protein
Specificity
For reactions to take place in physiological useful way, must occur at a rate that meets cell’s needs and must display specificity
Enzymes w/ similar functions may use different substrates
specificity = a particular reactant should always yield a particular product
Specificity of enzyme is due to the precise interaction of the substrate with the enzymes
Proteolysis Enzymes
- show the range of enzyme specificity
- catalyze the hydrolysis of peptide bonds
Substrate
reactants in enzyme-catalyzed reactions
6 Major Classes of Enzymes
- Oxidoreductases - transfer electrons between molecules (catalyze oxidation-reduction reactions)
- Transferases - transfer functional groups between molecules
- Hydrolyases - cleaves molecule by the addition of water
- Lyases – remove or add atoms to form double bonds
- Isomerases - move functional groups within a molecule
- Ligases - join two molecules in a reaction through the power of ATP hydrolysis
Enzyme Nomenclature
- Enzyme names have suffix (end) “ase”
- Enzyme naming is based on substrates and/or reactions
Ex. lactase – splits lactose into glucose and galactose
Ex. alcohol dehydrogenase – oxidizes or dehydrogenates ethanol
Cofactors
The catalytic activity of an enzyme may depend on Cofactors, which are small molecules (two types). Some enzymes require cofactors for optimal enzymatic activity
- Coenzymes
- Small organic molecule required for the activity of many enzymes
- derived from vitamins
- are typically loosely bound to the enzyme
- Tightly bound coenzymes are called prosthetic (helper) group - Metals
Apoenzyme
an enzyme w/o its cofactor (inactive)
Holoenzyme
an enzyme associated w/ its cofactor
will be in a configuration with a fully functional active site
catalytically active enzymes, consist of the protein components that form the main body of an enzyme (apoenzyme) and any other small cofactors
🔺G
difference between the reactants and the products
positive🔺G
endergonic reaction (requires energy, reaction won’t occur spontaneously)
negative 🔺G
exergonic reaction (releases energy, reaction will occur spontaneously)
When 🔺G = 0
at equilibrium
🔺G =🔺H - T🔺S where….
🔺G = gibbs free energy; change in free energy when a reaction proceeds
G = free energy: amount of energy available to do work
🔺 = change
🔺H = enthalpy or total energy
T = temperature in K (kelvins)
🔺S = entropy or unusable energy
It’s often difficult to determine the “true” 🔺G value. This can depend on….
- Temp
- pressure
- pH
- concentrations of products and reactants
Standard free energy change: 🔺G^o
What are the standard conditions?
- 298 K (25 C)
- products/reactants all at 1 M (molar concentration)
- pressures at 1 atmosphere
- to quantify standard free energy changes at pH 7, use 🔺G^o’ –> simple way to find this is to measure the concentrations of reactants and products when reaction has reached equilibrium
Equilibrium constant under standard conditions is:
K’eq = [C][D] / [A][B] where products are over reactants
🔺G^o’ = -RT ln K’eq
can be rearranged to:
K’eq = e^-🔺G^o’ /RT
K’eq = e^-🔺G^o’ /2.47
R is the gas constant (8.32J/mol K or 8.315 x 10 ^-3), T is 298 kelvins
Hydrolysis of ATP steps
There an ATP molecule. Water is added. This gives you ADP + pi (additional phosphate group)
ATP + H2O = ADP + Pi
The delta G naught prime for this reaction is -7.3 Kcal/mol mole or -30.5 kJ/mol. This is an exergonic reaction
Spontaneity rules
- A reaction can take place spontaneously only if 🔺G is negative
- A reaction cannot take place spontaneously if 🔺G is positive
- In a system at equilibrium, there is no net change in the concentrations of the products and the reactants and 🔺G is zero
- 🔺G of reaction depends only on the free energy of the products (the final state) minus free energy of reactants (initial state)
- The 🔺G of a reaction is independent of the path of the transformation
- 🔺G provides no information about the rate of a reaction
Enzyme reaction rates
enzyme can’t alter law of thermodynamics and can’t alter the equilibrium of a chemical reaction
reaction equilibrium is determined only by the free energy difference
reaction rate will be faster w/ enzyme
Enzymes facilitate the formation of the transition state
Enzymes lower transition states
ΔG‡ or activation energy
energy required to get to transition state
ΔG‡ = Gx^‡ - Gs
Transition state
molecule is no longer the substrate, but not yet the product
FACT: Chemical species that have the highest free energy and lowest concentration are those on the pathway from substrate to product
Enzyme Substrate Complex
The product of specific binding between the active site of an enzymes and its substrate
Active Site
A specific region of an enzyme that binds the enzyme’s substrate and carries out catalysis
Enzymatic Catalysis
- Begins w/ formation of an enzyme substrate complex. Enzymes bring substrates together to form an enzyme substrate complex on a particular region of the enzyme (active site)
- Interaction of enzyme and substrate promotes formation of the transition state
- The active site is a three-dimensional cleft or crevice created by amino acids from different parts of the primary structure of the protein
- The active site constitutes a small portion of the total enzyme volume or mass
- The active site creates a unique pocket or micro environment to facilitate the reaction
- The interaction of the enzyme and substrate at the active site involve multiple weak interactions
- Enzyme specificity depends on the molecular architecture at the active site
Enzyme Binding Energy
- Binding energy is the free energy released upon interaction of the enzyme and substrate
- Binding energy is greatest when the enzyme interacts w/ the transition state, thus facilitating the formation of the transition state
Enzyme Inhibitors
Enzyme inhibitors mimic the transition state; these can be called transition state analogs, which are potent inhibitors of enzymes
