Enzymes Flashcards
What are enzymes?
proteins that speed up (catalyse) specific chemical reactions
What are the properties of enzymes?
increase reaction rate
show specificity
are unchanged at end of reaction
do not alter reaction equilibrium
facilitate the reaction by decreasing the free energy of activation of the reaction (gibbs free energy)
- free activation energy is the difference between the transition state (energy required without a catalyst) and substrate free energy
- minimum activation energy is reduced
What is the active site of an enzyme?
the active site is a 3-D cavity or cleft that binds substrate(s) with specificity through electrostatic, hydrophobic, hydrogen bonding and Van der Waals interactions.
formation of an enzyme-substrate (E-S) complex at the active site is the first step in enzyme catalysis.
How are enzymes categorised? What is the function of proteases, nucleases, polymerases and kinases?
enzymes are categorised based on the reaction they promote
proteases - hydrolyse peptide bonds in proteins
nucleases - hydrolyse phosphodiester bonds between nucleotides in nucleic acids
polymerases - catalyse synthesis of polymers or nucleic acids
kinases - catalyses the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates = phosphorylation
What is the difference between the lock and key mechanism and the induced fit mechanism?
lock and key
- the enzyme active site is directly complementary to the substrate
induced fit
- the enzyme active site is not the same shape/complementary to the substrate
- as the substrate begins to bind to it, the active site changes shape to accommodate it and becomes complementary
What are the factors responsible for enzyme catalysis?
enzymes reduce free energy and speed up the reaction by using the enzyme-substrate binding energy to:
- to bring molecules together in the active site
- to constrain substrate movement
- to strain particular bonds in the substrate making breakage easier. substrate is distorted on binding to resemble transition state = means it needs less energy to reach the transition state
- to stabilise positive and negative charges in the transition state
- to exclude water from the active site = makes the reaction go faster
- to provide a reaction pathway of lower energy = involving covalent enzyme-substrate intermediates
- use cofactors = bring new chemistry to the active site with NAD(H), FAD(H2), metal ions such as Mg2+
What is competitive inhibition?
inhibitor competes with the substrate for binding to the active site
- inhibitor forms an inactive E-I complex
- inhibitor blocks the substrate from binding
Km is increased
- lower enzyme affinity for substrate = fewer E-S complexes
- requires more substrate to achieve Vmax/2
Vmax is unaffected = same y-intercept
- inhibitor can be competed out by high concentration of substrate
What is non-competitive inhibition?
inhibitor binds at the allosteric site = not at the active site
- does not compete with the substate for active site binding
- enzyme can be bound to both (E-S-I complex) but cannot form products
Km is unaltered
Vmax is reduced
- cannot be competed out by increasing substrate concentration
How can enzyme activity be regulated?
control of gene expression
- controls enzyme amount
compartmentation
- sequences in enzyme polypeptide chain target enzymes to organelles = makes sure the enzymes goes to where it is needed
covalent modification of the enzyme
- changes enzyme shape and activation = phosphorylation
allosteric regulation
How does allosteric regulation regulate enzyme activity?
allosteric regulation
- is a regulatory molecule that acts at a pocket distinct/different from the active site
- changes the enzyme conformation to influence the active site and decrease (or in some cases, increase) enzyme activity
- controls the flux of material through a metabolic pathway
What is feedback inhibition?
reaction product used to regulate its own production
- end product is a different shape to the starting reactant and regulates the reaction
