B1-4 Enzymes Flashcards
Definition of Enzymes
Biological catalysts that possess the ability to speed up a reaction without being themselves changed at the end of a reaction
How do enzymes work (not the hypotheses ah don’t think so far)
Enzymes increase the rates of chemical reactions by lowering the free energy barrier that separates the reactants and products i.e. activation energy
Definition of Activation energy (Ea)
Minimum amount of energy required to start a chemical reaction
Lowering of Ea
At physiological temperatures, few molecules can overcome Ea barrier and can only do so with the use of a catalyst
Activation energy is lowered by enzyme
More reactant molecules can surmount the energy barrier to reach the transition state to be converted into product molecules
Total energy difference/ free energy change/ Gibbs free energy change between reactant and product molecules remains the same
Properties of Enzymes (6)
Effective in small amounts (chemically unaltered, reusable)
Extremely efficient
High degree of specificity
Denatured by heat and act most efficiently at optimum T
Affected by pH and act most efficiently at optimum pH
Activity can be regulated by activators and inhibitors
Structure of enzymes
Most are globular proteins
Have specific 3D conformation necessary for their action
3D structure must be maintained for enzyme to remain functional
An enzyme can be denatured when bonds holding them in specific 3D conformation are disrupted
4 categories of aa residues
Catalytic aa residues - R groups involved in catalytic activity
Binding aa residues - R groups hold substrate in position via covalent bonds
Structural aa residues - Maintain specific 3D conformation of active site and enzyme as a whole
Non-essential aa residues - No specific function, can be removed or replaced without loss of enzyme’s catalytic function
Cofactor - Inorganic metal ions
Usually small and divalent (charge +2)
Component of active site or allosteric regulator
Bind reversibly to enzyme and act by altering enzyme’s active and/or allosteric sites to facilitate catalytic reaction carried out by enzyme
Cofactor - Coenzyme
Loosely associated with enzyme during reaction
Act as transient carriers of specific functional groups, hydrogens, or electrons
Most are derived from vitamins
Cofactor - Prosthetic Group
Tightly bound to enzyme on permanent basis
Allosteric Enzymes
Alternate between active and inactive form
Have multiple subunits and through conformational changes, binds activators of inhibitors at sites other than active site
How Enzymes lower Ea
Orientate S in close proximity, in correct orientation, to undergo chemical reactions
Strain critical bonds in S molecule(s), allowing S to attain unstable transition state
Provide microenvironment that favours reactions
Lock and Key Hypothesis
There is an exact fit/complementary shape or conformation between S and active site of E
E is viewed as a rigid structure, where only S exactly complementary to conformation of active site are able to bind to active site for catalysis
Explains substrate specificity of enzymes
Induced Fit Hypothesis
Active site conformation is not precisely complementary to that of S before binding
Active site is not a rigid conformation that fits only one type of substrate
Upon binding, active site of E changes conformation slightly to bind S even more firmly so that R groups of catalytic aas at active site are moulded into specific conformation and brought into close proximity to the chemical bonds in S to facilitate catalysis
Further explains group specificity where one enzyme is able to catalyse reactions for a variety of substrates that share similar structural or chemical properties
Rate of enzyme-catalysed reaction and how to measure
Rate is the amount of S which is converted to P by enzymes per unit time
Measured by rate of product formation or substrate usage
Competitive Inhibitor
Structurally similar to S, compete with S for binding to active site
Although it is not acted upon by E, it remains bound to the active site and prevents S binding to active site
Initial rate reduced, but both reactions reach same Vmax
Requires longer time in presence of inhibitor to reach same Vmax
Km higher for inhibited
Why increase in S conc reduces effect of competitive inhibition
Because S and inhibitor are in direct competition for E’s active sites
Greater proportion of S - Greater chances of out-competing inhibitor - Rate of reaction almost equivalent to Vmax can be attained
Final amount of P formed same as S continues to be converted by E molecules unaffected by inhibitor
Non-competitive Inhibitor
Bears no structural resemblance to substrate, does not compete with S for active site
Binds to part of E that is not active site
Binding of inhibitor alters 3D conformation of E and active site
E molecule no longer has active site complementary in conformation to S
Hence S does not bind to E active site and no E-S complex can be formed
Why increase in S conc reduces effect of non-competitive inhibition
Even when substrate concentration is very high, initial rate of reaction does not reach same Vmax as uninhibited reaction
Binding of non-con to site other than active site causes change in 3D conformation of enzyme active site that prevents S from binding
Why non-competitive inhibitor has lower Vmax
Certain proportion of E molecules are rendered inactive
As S and inhibitor are not in direct competition for same site, increase in S concentration has no effect on inhibition
Km remains unchanged as affinity of enzyme for substrate remains unaffected
Final amount of P formed same as S continues to be converted by E molecules unaffected by inhibitor
Why non-competitive inhibitor has lower Vmax
Certain proportion of E molecules are rendered inactive
As S and inhibitor are not in direct competition for same site, increase in S concentration has no effect on inhibition
Km remains unchanged as affinity of enzyme for substrate remains unaffected
Final amount of P formed same as S continues to be converted by E molecules unaffected by inhibitor
Allosteric Regulation
Regulation of an E by binding of molecules (regulators: activators, inhibitors) at an allosteric site i.e. site other than active site
Allosteric enzyme structure
Most allosterically regulated E are composed of 2 or more polypeptide chains - Multi-subunit E
Each subunit has its own active site and allosteric sites are usually located where subunits are joined
Allosteric Inhibition
Binding of activator to active site induces favourable conformation change in active site of all subunits of enzyme
This significantly amplifies response of E to substrates. The amplification results in sudden steep rise in rate of reaction
