Enzymes Flashcards
What are enzymes?
Biological catalysts that increase rate of reaction without being created or destroyed
In which direction do enzymes catalyse reactions?
Both directions, don’t change equilibrium but change the rate at which equilibrium is reached
How is the speed of an enzyme quoted?
In terms of its ‘turnover number’ or ‘catalytic constant’
What is the catalytic constant (Kcat)
The number of molecules of ‘substrate’ that one enzyme molecule can convert in one second GIF substrate concentration is saturating)
What are the 6 main groups of enzymes?
- Oxidoreductases
- Transferases
- Hydrolases
- Lyases
- Isomerases
What is the type of reaction for oxidoreductases?
Transfer of electrons (hydride ions or H atoms)
Give an example of a process that involves oxidoreductases
Respiration
What is the type of reaction for transferases?
Group transfer reactions
Give an example of an ion that is frequently transferred in biological reactions
Phosphate - in respiration it is added to substrates to make them more reactive
What is the type of reaction for hydrolases?
Hydrolysis reactions - Water cleaves a bond
Give an example of an enzyme involved in hydrolysis reactions?
Amylase - hydrolysis of starch
What is the type of reaction for lyases?
Addition of groups to double bonds (or the reverse)
Give an example of an enzyme involved in the formation of a double bond
Aldolase - in GAP during glycolysis
What is the type of reaction for isomerases?
Transfer of groups within molecules to yield isomeric forms
Give an example of an enzyme involved in catalysing interconversion
Triose phosphate isomerase - inter conversion of DHAP and GAP during glycolysis
What is the type of reaction for ligases?
Formation of C-C, C-S, C-O and C-N bonds by condensation reactions
Give an example of an enzyme involved in the formation of C-C bond
Pyruvate carboxylate - between pyruvate & Carbon dioxide to form oxaloacetate at the start of TCA cycle
How do enzymes work?
- They lower activation energy
- substrate binds in active site of enzyme
What is an exergonic reaction?
A reaction that releases energy
What is an endergonic reaction?
A reaction the uses energy
Which type of reaction is favourable for enzymes: exergonic or endergonic?
Exergonic
What is activation energy?
The minimum amount of energy needed by molecules in order for them to react
What are active sites complementary to?
The transition state, NOT the substrate
What is induced sit?
When enzymes undergo a conformational change upon substrate binding
What are the 5 catalytic strategies?
- Catalysis by approximation
- Covalent catalysis
- General acid-baes catalysis
- Metal ion catalysis
- Cofactors
What is catalysis by approximation?
When 2 substrates are brought together on a single binding surface on an enzyme
What is covalent catalysis?
The active site contains a reactive group that becomes temporarily covalently attached to a part of the substrate
What is general acid-base catalysis?
A molecule other than water plays the role of a proton donor or acceptor
What is metal ion catalysis?
Metal ions can mediate oxidation-reduction reactions by reversible changes in the metal ion’s oxidation state or stabilise substrate binding
What are cofactors?
Inorganic ions or coenzymes which are required for enzyme activity
What are cofactors?
Inorganic ions or coenzymes which are required for enzyme activity
What is a prosthetic group?
When a metal ion or an organic compound is covalently bound to a protein and is essential for its activity
What is a haloenzyme?
A catalytically active enzyme, including all necessary subunits, prosthetic groups and cofactors
What is an apoenzyme
The protein portion of an enzyme, exclusive of any organic or inorganic cofactors or prosthetic groups
What affects the rate of an enzyme catalysed reaction?
- Substrate concentration
- Enzyme concentration
- pH
- Temperature
How does substrate concentration affect the rate of an enzyme catalysed reaction?
Enzyme activity increases with increasing substrate concentration until enzyme concentration becomes limiting.
The maximum velocity of an enzymatic reaction when the binding site is saturated with substrate is called Vmax.
How does enzyme concentration affect the rate of an enzyme catalysed reaction?
Enzyme activity increases with increasing enzyme concentration.
The maximum velocity of an enzymatic reaction, Vmax, is increased with increasing enzyme concentration.
How does pH affect the rate of an enzyme catalysed reaction?
pH affects the protonation state of amino acid side chains.
Protonation state is critical for side chains in the active site involved in acid-base
catalysis and changes away from the optimum pH will have a negative effect.
The protonation state of side chains involved in stabilising tertiary structure is also critical. Changes in pH will break bonds and cause protein unfolding. Denaturation.
How does temperature affect the rate of an enzyme catalysed reaction?
Heat provides energy for molecules to move, hence moderate increases in temperature will increase reaction rates due to increased molecule collisions.
Significant increases in temperature increase vibrations in the enzyme causing bond breakage.
What are the types of enzyme inhibition?
Reversible inhibition: - Competitive - Uncompetitive - Noncompetitive Irreversible inhibition
What is competitive inhibition?
Inhibitor is structurally similar to substrate
and competes with the substrate for the E active site.
Once in the active site, prevents substrate binding.
What is uncompetitive inhibition?
Inhibitor binds to ES complex only, at site distant from active site
This may result in a conformational change within the enzymes structure – inactivating it.
What is noncompetitive inhibition?
Also binds at a site distant from the active site, but can bind both E and ES.
What is irreversible inhibition?
Inhibitors covalently bond to the enzyme – especially to key functional groups within the enzymes active site.
How are metabolic pathways regulated?
By feedback inhibition which ensures that the amount of final product does not exceed what is needed and hence resources are not wasted.
