Enzymes Flashcards
(37 cards)
What is an enzyme?
• Globular protein
• Biological catalyst that differs from a chemical catalyst in that they;
– Catalyses very high reaction rates
– Shows great reaction specificity
– Work in mild temperature/pH conditions
– Can be regulated
• Very important – nearly all biochemical reactions involve them
What is a ribozyme?
Some enzymes are termed ribozymes – catalytic RNA molecules with no protein component
What is a cofactor?
Non-protein component needed for activity
E.g. Fe2+ or Fe3+, K+ and Mg2+
What is a coenzyme?
Complex organic molecule, usually formed from vitamins
E.g. FAD, NAD+, Coenzyme A
What is a prosthetic group?
Cofactor covalently bound to the enzyme or very tightly associated with the enzyme
What is a apoenzyme?
The protein component of an enzyme that contains a cofactor
What is a holoenzyme?
“whole enzyme” – the apoenzyme plus the cofactor(s)
What is a substrate?
Molecule acted on by the enzyme
What is the active site on an enzyme?
Part of the enzyme in which the substrate binds and is acted upon
Name the 6 classes of enzymes given by the IUBMB
Oxidoreductases Transferases Hydrolases Isomerases Lyases Ligases
Enzymes are OTHILL
How to enzymes affect reactions?
They do NOT:
– Move reaction equilibria
– Make a non-spontaneous reaction spontaneous
They DO:
– Increase rates of spontaneous reactions
– Lower the activation energy of biochemical reactions
– Accelerate movement towards reaction equilibrium
What is free energy and how can it be calculated?
“Useful” energy generated from cellular reactions is termed Gibbs Free-Energy (G), originally called “available energy”.
Spontaneous reactions must have a –ve ΔG value as they will decrease enthalpy (H) and/or increase entropy (S)
ΔG = ΔH - TΔS
How do enzymes reduce activation energy?
- Entropy reduction, physically unite molecules
- Desolvation, replaces bonds between water and substrate
- Induced fit
How can you scientifically analyse an enzyme?
o Enzyme kinetics – information on substrate and product affinity to the enzyme, knowledge of the dynamic properties of enzyme catalysis is useful in drug design
o Mutagenesis – useful in the study of enzyme effects, identification of the active site, and the design of novel proteins
o 3D structure – good for identifying structure, function if uncharacterized, interactions with other macromolecules, structure based drug design etc.
Define the ‘Michaelis Constant’, describe how it may be experimentally estimated and describe what knowledge of the constant may allow to be deduced about the nature of the catalysed reaction.
Km = ½ Vmax = k-1 + k2 / k1
Larger Km values indicate a less stable ES complex.
Lower Km values indicate a more stable ES complex.
Vmax also tells us the degree of activity of an enzyme.
Therefore, it tells us about the affinity of the enzyme for the substrate.
What does Km represent?
“Km is equivalent to the substrate concentration at which the initial reaction rate is half of the maximum reaction rate”
What are isoenzymes?
Different enzymes that catalyse the same reaction.
E.g. hexokinase (muscle) and glucokinase (liver; aka hexokinase D) both phosphorylate glucose to G6P, but have different Km for glucose
What is gel electrophoresis?
Gel electrophoresis is a laboratory method used to separate mixtures of DNA, RNA, or proteins according to molecular size.
DNA is negatively charged, therefore, when an electric current is applied to the gel, DNA will migrate towards the positively charged electrode. Shorter strands of DNA move more quickly through the gel than longer strands resulting in the fragments being arranged in order of size.
Whats an allosteric enzyme?
Allosteric enzymes are enzymes that change their conformational ensemble upon binding of an effector, which results in an apparent change in binding affinity at a different ligand binding site.
What can affect enzyme function?
Temperature
pH
Inhibitors
What is a competitive inhibitor?
- These bind to enzymes non-covalently and will usually resemble the substrate molecule, therefore competing with the active site
- This leads to a decrease in the affinity between the active site and the substrate, so the Km of the substrate-enzyme complex increases
- Increasing substrate concentration can overcome this inhibition, so the same Vmax can be achieved
What is a non-competitive inhibitor?
- These bind to enzymes non-covalently and will usually attach to a site other than the active site of the enzyme
- The substrate is usually still able to bind the active site, so the Km of the substrate-enzyme complex remains unchanged
- Increasing substrate concentration does not change the inhibition so the Vmax will decrease
What is an irreversible inhibitor?
• This type of inhibitor will usually bind to the enzyme in a covalent, and therefore irreversible way
Describe the two models that explain allosteric enzyme kinetics
The concerted model
The sequential method
