Enzymes Flashcards
what are enzymes
biological catalysts which speed up rate of reaction, by lowering activation energy, without altering the final equilibrium of reactants and products
how do enzymes affect the final equilibrium of reactants and products
they dont; they remain unchanged
how do enzymes speed up rate of reactions
by lowering the activation energy required for reaction
roles of enzymes
- speed
- selectivity
- specificity
how enzymes affect speed
- enhance rate of reaction 10^6 - 10^14 times compared to uncatalysed reaction
- some reactions are so fast they are only limited by the diffusion rate of substrate to active site
how do enzymes affect specificity
enzymes can be very specific
e.g
- glucosidase will hydrolyse glucose from an oligosaccharide, but will not hydrolyse galactose from a oligosaccharide
- glucose and galactose differ at only one chiral centre on the opposite side of the ring from point of hydrolysis
= great specificity
how do enzymes affect selectivity
enzymes will often catalyse only a single reaction and carry out that reaction stereoselectively
e. g
- a particular glucosyl transferase will only add glucose on to the 2 postition of another glucose
- it will not add glucose to the 3, 4, or 6 positions
effect of enzymes on activation energy of a reaction
enzymes help overcome the activation energy barrier
even if a reaction is energetically favourable (i.e goes from high to low free energy), there is till Ea to overcome
how do enzymes lower activation energy
- often by taking a different route to get to the same destination
- this may not occur via a simple transition state
- there can be multiple intermediates that might not be present inthe uncatalysed reaction
Rate limiting step
- always the highest transition energy COMPARED to the relative intermediate
- this means it is not always the highest transition energy
- look for segments where intermediate is lower in energy that intial reactants
- will be the slowest step of the reaction
what is a key property of enzymes
defined 3D structure which enables binding of substrate in presence of certain AA, when arranged in a certain way which facillitae bindng at the active site
what is the active site
can be on the surface or inside an enzyme
location of chemical binding
steps of enzyme action
- enzyme is active
- specific substrate with correct stereospecificity will bind
- enzyme-substrate-complex forms
- catalysis occurs
- enzyme-product-complex forms
- product is released
- enzyme is recycled
why does the product leave the enzyme
the product typically has a lower affinity for the enzyme than the substrate.
favourable characteristic as allows recycling of enzyme and product to be produced
substrate specificity
- often enzymes will catalyse one type of reaction
e.g alcholol dehydrogenase willl oxidise primary alcohols to aldehydes
this is an example of group specificty because the enzyme acts on all primary alcohols - very few enzymes only act on one substrate
stereospecificity
- if a substrate occurs naturally in two steriosomer forms, the enzyme concerned with its metabolism in the cell will usually only act on one isomer
- hence some forms of subsrates are considered active and inactive
what determines specificity
presence of the active site, into which only the substrate with the correct shape and charge will fit
what has enzyme specificty lead to
systematic classification scheme
systematic classification scheme of enzymes
- 6 main classes according to type of reaction they catalyse
- each class is split into subgroups according to their substrate or source
- each enzyme is identified by its own 4-digit number
6 classes of enzymes
- oxidoreductases
- transferases
- hydrolyses
- lyases
- isomerases
- ligases
type of reaction and example enzyme for: oxidoreductases
- add O2 or remove H2
- Lactate dehydrogenase
= pyruvate -> lactate (by action of NADH)
type of reaction and example enzyme for: transferases
- transfer of functional groups
- alanine amino transferase
= gulatmate+pyruvate -> alpha-ketoglutarate + L-alanine
type of reaction and example enzyme for: hydrolases
- hydrolytic reactions
- Trypsin
type of reaction and example enzyme for: lyases
- add groups to -C=C bonds
- ATP-citrate lyase
= citrate -> oxaloacetate + acetate