6.5 Enzymes and the Rate of Chemical Reactions Flashcards
What are catalysts?
substances that increase the rate of chemical reactions without themselves being consumed (ie. enzymes)
What do enzymes play a critical role in?
determining which chemical reactions take place from all the possible reactions that could occur in a cell
What do enzymes do?
reduce the activation energy of a chemical reaction by stabilizing the transition state and decreasing its free energy
What are exergonic reactions?
release free energy
What are endergonic reactions?
require free energy
What do all chemical reactions require intially?
an initial input of energy is needed to proceed
Why is an input of energy needed for all chemical reactions?
as a chemical reaction proceeds, existing chemical bonds break and new bonds form
What is a transition state?
brief time in a chemical reaction in which chemical bonds in the reactants are broken and new bonds in the product are formed
it is highly unstable and therefore has a large amount of free energy
all chemical reactions have at least one transition state before they convert into products
How does a reactant reach a transition state?
must absorb energy from its surroundings
as a result, all chemical reactions, even spontaneous ones that release energy, require an input of energy that we can think of as an “energy barrier”
What is activation energy?
energy input necessary to reach the transition state
What happens once the transition state is reached?
reaction proceeds, products are formed, and energy is released into the surroundings
What is the correlation between rate of reaction and height of energy barrier?
an inverse correlation between the rate of a reaction and the height of the energy barrier: the lower the energy barrier, the faster the reaction and the higher the barrier, the slower the reaction
What does not change although an enzyme reduces the activation energy?
the difference in free energy between reactants and products (ΔG) does not change
an enzyme changes the path of the reaction between reactants and products, but not the starting or end point
How do enzymes emerge from a chemical reaction unchanged?
they form a complex with the reactants and products
What is a substrate?
reactant in a chemical reaction catalyzed by an enzyme
Describe
S + E ⇋ ES ⇋ EP ⇋ E+P
in the presence of an enzyme:
- substrate forms a complex with enzyme (ES)
- still part of the complex, substrate is converted to product (EP)
- complex dissociates, releasing the enzyme and product
What is the formation of a complex critical for?
accelerating the rate of a chemical reaction
What is an active site?
portion of an enzyme that binds substrate and converts it to product
How is the active site formed?
enzymes are folded into three-dimensional shapes that bring particular amino acids into close proximity
What bonds are formed in the active site? What do these interactions do?
enzyme and substrate form transient covalent bonds, weak noncovalent interactions, or both
together, these interactions stabilize the transition state and decrease the activation energy
How do enzymes reduce the energy of activation?
by positioning two substrates to react: within the active site, their reactive chemical groups are aligned and their motion relative to each other restrained
Why are enzymes so large? Of the many amino acids that form the active site, why are only a few actively contributing to catalysis?
each of these amino acids has to occupy a very specific spatial position to align with the correct reactive group on the substrate
if the few essential amino acids were part of a short peptide, the alignment of chemical groups between the peptide and the substrate would be difficult or even impossible because the length of the bonds and the bond angles in the peptide would constrain its three-dimensional structure
catalytic amino acids are spaced far apart in the primary structure of the enzyme, but brought close together by protein folding
How big is the active small?
extremely small compared with the size of the enzyme
Why is the large size of many enzymes required?
to bring the catalytic amino acids into very specific positions in the active site of the folded enzyme
Why are enzymes highly specific?
- they recognize either a unique substrate or a class of substrates that share a common chemical structure
- catalyze only one reaction or a very limited number of reactions
What can enzyme activity be influenced by?
inhibitors and activators
What are inhibitors?
- synthesized compound that decreases the activity of an enzyme
- very common
- synthesized naturally by many plants and animals as a defence against predators
- many drugs to treat infections and diseases use them
What are activators?
compound that increases the activity of an enzyme
What are the 2 classes of inhibitors?
irreversible: usually form covalent bonds with enzymes and irreversibly inactivate them
reversible: form weak bonds with enzymes and therefore easily dissociates from them
Describe 2 different ways inhibitors can act.
- inhibitor is similar in structure to the substrate and therefore is able to bind to the active site of the enzyme
- binding of the inhibitor prevents the binding of the substrate
- inhibitor competes with the substrate for the active site of the enzyme
- can often be overcome by increasing the concentration of substrate - usually has a structure very different from that of the substrate
- bind to a site other than the active site of the enzyme but still inhibit the activity of the enzyme
- in this case, binding of the inhibitor changes the shape and therefore the activity of the enzyme
What are allosteric enzymes?
enzymes that are regulated by molecules that bind at sites other than their active sites
What is the activity of allosteric enzymes influenced by?
both inhibitors and activators
What do allosteric enzymes play a key role in?
regulation of metabolic pathways
What is negative feedback?
- effect in which the final product of a biochemical pathway inhibits the first step
- the process in which a stimulus acts on a sensor that communicates with an effector, producing a response that opposes the initial stimulus
- negative feedback is used to maintain steady conditions, or homeostasis
