Enzymes Flashcards
Enzymes: Powerful Catalysts
An enzyme is a biological catalyst
Nearly all enzymes are proteins
Allow reactions to occur under mild (physiological) conditions
Highly specific
Subject to regulation
Enzymes: Highly Specific
Specific both in reactions they catalyse and their choice of reactant or substrate
Catalyse a single reaction or a series of closely related chemical reactions
Example: proteolytic enzymes – hydrolyse peptide bonds
Class 1
Class 1: OXIDOREDUCTASES:
Catalyse oxidation-reduction reactions
These enzymes transfer electrons between molecules
Example: Lactate Dehydrogenase
Class 2
Class 2: Transferases:
Catalyse transfer of functional groups between molecules
Aminotransferases (a class of transferases) shuffle amine groups between donor and acceptor molecules
Example: Alanine Transaminase
Class 3
Class 3: Hydrolyases
Catalyse hydrolysis by cleaving molecules by the addition of water
Trypsin is an example (seen earlier) and another is:
Example: Pyrophosphatase
Class 4
Class 4: Lyases
Catalyse addition of atoms or functional groups to a double bond OR removes them to form a double bond
Example: Fumarase
Class 5
Class 5: Isomerases
Catalyse movement of functional groups within a molecule
Generally simplest enzymatic reactions as only 1 substrate and 1 product
Example: Alanine racemase
Class 6
Class 6: Ligases
Catalyse bond formation or joining two molecules at expense of ATP
An example is DNA ligase or another one is:
Example: Glutamine synthetase
Co-Factors
Catalytic activity of many enzymes require co-factors
Enzyme without its co-factor is termed an apoenzyme
Complete catalytically active enzyme is a holoenzyme
Co-factors:
(1) small organic molecules derived from vitamins and called coenzymes and
(2) metals
Tightly bound coenzymes are called prosthetic groups
Enzymes: Examples of Co-Factors
Coenzyme
Thiamine pyrophosphate (TPP) Pyruvate dehydrogenase
Flavin adenine nucleotide (FAD) Monoamine oxidase
Nicotinamide adenine dinucleotide (NAD) Lactate dehydrogenase
Coenzyme A (CoA) Acetyl CoA carboxylase
Metal
Zn2+ Carbonic anhydrase
Ni2+ Urease
Se Glutathione peroxidase
K+ Acetoacetyl CoA thiolase
Enzymes: Reaction Rate
Enzymes do not alter the equilibrium of a chemical reaction
Same equilibrium point is reached but more quickly in the presence of the enzyme
Same amount of product is produced, just produced more quickly
Enzymes: Transition State
How do enzymes accelerate how quickly this equilibrium is attained?
S <> X‡ > P
X‡ is called a Transition State – it has higher free energy than S or P
Enzymes: Activation Energy
Enzymes facilitate the formation of the transition state
P + Q have energy below A + B
Substrate and enzyme combine to create a pathway whose transition-state energy is lower than when enzyme is not present
More molecules have required energy to reach transition state and thus more product is formed faster (BUT NOT ANY MORE PRODUCT)
Transition state is only transient and unstable due to high free energy contained within it
Enzyme inhibitors based on TS are difficult to synthesise because they are unstable and difficult to synthesise
active site
the active site is the region of an enzyme that binds substrates
it is the interaction of the enzyme and the substrate at the active site that promotes formation of the transition state
the active site is
a 3D cleft or crevice
small part of the total volume of the enzyme
unique microenvironment
substrates bound to enzymes by multiple weak attractions
binding specify depends on precisely defined arrangements of atoms in the active site - substrate must have matching shape to activate site
substrate binding to enzyme
the active sites of enzymes assume a shape that is complementary to that of the substrate only after the substrate has been bound
this leads to conforming the shape of the active site to the shape of the substrate in its transition state
this is known as induced fit
