Intro to Enzymes Flashcards
What are enzymes?
- Biological catalysts of chemical reactions
- Specific action on particular biochemical compounds
- Nearly all are proteins
- Increase rate at which equilibrium is reached
What happens at equilibrium?
Rate of forward and back reactions is equal
how do enzymes increase rate of reaction?
by lowering the activation energy required
How do enzymes decrease Ea?
- By providing catalytically competent groups for a specific reaction mechanism
- Binding substrates such that their orientation is optimised for the reaction
- By preferentially binding and stabilising the transition states of the reaction
What is the active site of an enzyme?
- is the region of the enzyme at which substrate binding and conversion to product takes place
- is a 3-dimensional space comprising crucial amino acid residues
- may represent only a small part of the protein structure
- binds substrate via multiple weak interactions
- provides substrate specificity because of unique 3d arrangement
Example of an active site
Trypsin
- Proteolytic digestive enzyme
- Three key amino acids in active site (His, Asp, Ser) but far apart in amino acid sequence
- Active site is “niche” for binding substrate
What is the K1?
rate constant of formation enzyme- substrate complex
What are K2 & K3?
rate constants of dissociation of ES complex
What is V?
Rate (velocity) of formation of product
What is Vmax?
Theoretical maximum rate of reaction (all enzyme saturated with substrate)
What is [s]?
Substrate conc
What is Km?
Michaelis constant
When plotting V against [S] what happens?
enzyme activity increases with increased substrate conc
Vmax is a theoretical value that cannot be achieved in the test tube
What types of inhibition are there?
Reversible (non-covalent)
Irreversible (covalent)
What types of inhibitors are there?
competitive or non-competitive
What is irreversible enzyme inhibition?
Covalent modification of the enzyme, often at amino acid side chains in the active site (orthosteric)
What are 2 examples of irreversible enzyme inhibition?
- Carboxymethylation of cysteine side chains by iodoacetamide
- Modification of active site serine residue of acetylcholine esterase (AChE) by nerve agents
What is reversible inhibition?
Reversible inhibition mechanisms
Competitive & non-competitive
What is Competitive inhibition?
- Either inhibitor or substrate can bind to enzyme
- Inhibitor usually binds at the orthosteric site
- Inhibition can be overcome by high substrate concentration
What does competitive inhibitor concentration affect?
Affects Km but not Vmax
What is Non- competitive inhibition?
- Inhibitor and substrate can bind simultaneously
- Binding occurs at allosteric site
- Inhibitor alters conformation or accessibility of active site
- Inhibition not affected by high substrate conc
What does non- competitive inhibitor concentration affect?
Affects Vmax but not Km
What is the arachidonic acid pathway?
Cyclooxygenases (COX) transform arachidonic acid into prostaglandins (inflammatory mediators)
What is the importance of COX?
COX isoforms COX-1 & COX-2 have different tissue distribution and function
Non- steroid anti-inflammatory drugs (NSAIDS) have different selectivity for COX-1 vs COX-2
