Enzymes Flashcards
enzymes
biological catalysts, speed up rate of reaction without being used up in the process
highly specific
enzyme structure
based on primary, secondary and tertiary structure
globular protein
amino acid sequences specifies 3d conformation
active site made up of binding site (bind and orient substances) and catalytic site (reduce activation energy)
enzyme functions
metabolism, movement, digestion, coupling unfavourable reactions, cell signalling, gene expression
enzymes in disease
malfunction in enzyme activity can disrupt homeostasis
dna mutations
single amino acid sub could destabilise protein stucture
PKU- disease example
genetic disorder- mutation in phenylalanine hydroxylase
reduced metabolism of phenylalanine
build up in blood/ brain is toxic
active site
active site is highly specific
forms a crevice on surface of enzyme
substrate enters active site
bound by weak forces incl hydrogen bonding, electrostatic forces
enzyme binding
lock and key- considers it is rigid and fixed
induced fit- conformational change when binding
substrate specificity
amino acids residues determine substrate specificity
complementary shape, charge and hydrophobic/phillic interactions
highly selective for substrate
allosteric site
away from active site
binding to allosteric site can induce conformational change (change in ror)
mechanism of regulation
activate or inhibit reactions
acts as feedback mechanism
enzyme co factors
some enzymes require co factors for optimal activity; in order to catalyse reaction
inorganic molecules/ organic compounds
coenzyme- transiently bound, may be altered during reaction, often vitamin precursors (deficient diseases if inadequate)
prosthetic groups- metal/ coenzyme covalently bonded (not altered during reaction)
oxidoreductases
transfer of oxygen or hydrogen atoms or electrons from one substrate to another
lysozyme
abundant in secretions, anti bacterial defence
cleaves glycosidic bonds of peptidoglycan
when increasing RoR
E+S- binding = ES complex- catalysis = E+P
enzymes only alter the rate of reaction, not amount of product formed
product formation levels off= equilibrium
biological reactions
spontaneous reactions can only occur if free energy of a system is negative (exergonic)
input of energy is required if ΔG is positive (endergonic)
enzyme reactions
all reactions need to overcome the energy barrier (activation energy)
enzymes reduce activation energy required for the reaction to proceed
transition state- transient molecule state that is no longer substrate but not yet product
types;
exergonic= spontaneous
endergonic= unfavourable
coupled reactions
exergonic= spontaneous
endergonic= unfavourable ( need a reaction coupled)
reducing activation energy
influences
microenvironment eg pH
orientation
hydrophobic interactions
ionic/hydrogen/ covalent bonds
catalytic mechanisms
catalysis by approximation- bringing the reactants closer
covalent catalysts- chymotripsin (formation of covalent bond- share an electron- between enzyme and substrate)
acid base catalysis- H+, -OH- adding acid/base helps to catalyse reaction
metal ion catalysis- zinc, ion- enzyme requires cofactor for reaction to proceed
enzyme velocity (V0)
rate of reaction= V0- at time zero/first 10% of substrate (umol/min)
=amount of substrate converted to product per unit of time
reported as values at time zero (or before 10% of S is converted) to ensure reaction is at fastest rate, least amount of P, highest S concentration
substrate conc affects initial velocity, doubling S will double V0- if enzymes are saturated there will be no increase in V0
reaction rate dependent on P dissociation from enzyme- double E will double V0
michealis-mentin kinetics
on conc against ror graph
Vmax- maximum RoR
half Vmax extrapolate down to subsrate conc= Km
Km= measure of enzyme affinity for substrate (michealis constant)
measured in units of conc- molar (M)
determined experimentally- conc of S is where velocity = half of Vmax
high Km- weak binding (low enzyme affinity)
low Km- strong binding (high enzyme affinity)= greater V0= faster RoR
temperature
rise in temp;
- increases thermal energy to overcome activation energy
- beyond optimal will have rapid decrease in reaction rate, breaking weak bonds, altering active site and denaturing protien structure
pH
diversity of optimal pH conditions around the body
small deviations will result in reduction in activity
-ionisation of groups in active site
large deviations lead to denaturation
enzyme assays
monitiors the dissapearance of substrate or appearance of product
can be measured change in colour/ absorbance, chromotography, radiography etc
can be used to measure enzyme kinetics using initial rate (V0) and time course (Vmax and Km)
substrate analogues
chemical compounds similar in structure to substrate + bind to active site eg poison, drug, antibiotic etc
in vivo- involved in regulation as inhibitors of enzymes
many drugs that treat disease act via enzyme inhibition
