Introduction to Enzymes in Physiology Flashcards
What is an enzyme?
- Enzymes are biological catalysts
- Provides a reaction surface (active site)
- Bring reactants together
- Position reactants correctly for reaction
- Weaken bonds in the reactants
- May provide acid/base catalysis
- May provide nucleophiles
- Do not change equilibrium position of reaction
Describe enzyme energetics.
- For a chemical reaction to proceed it must exhibit a negative ΔG
- ΔG = free energy of initial state - free energy of final state
- To reach final state reaction must first reach activation energy
- Enzymes decrease reaction energy = increased rate of reaction
Describe what occurs at the enzyme active site.
- Region within an enzyme that fits the shape of substrate molecules
- Amino acid side-chains align to bind the substrate through H-bonding, hydrophobic interactions etc
- Products are released when reaction is complete
Describe the Lock and Key hypothesis.
- Active site has rigid shape
- The fit between substrate and active site is exact
- Temporary structure called enzyme-substrate complex formed
- Products have different shapes from substrate and release from active site
Describe the induced fit hypothesis.
- Active site is flexible and can change its conformation
- Shape of enzyme, active site and substrate adjust to maximise fit
- Greater range of substrate specificity, reacting with substrates of similar type
What is the optimum pH for enzymes?
~7.4
- Activity is lost at low or high pH as tertiary structure is disrupted
What happens to enzymes when there are changes to pH?
- Alters the ionisation state of amino acid side chains
- Alters ionic bonding, structural stability, shape and functionality of enzyme
- Related to the overall 3D structure of all enzymes
How does temperature affect enzyme activity?
- Enzymes most active at optimum temp of 37°C
- Show little activity at low temp.
- Activity is lost at high temp as denaturation occurs
What is the function of cofactors?
Bind to enzymes to maintain correct configuration of active site - metal ions and other inorganic factors
What are coenzymes?
- An organic molecule bound to enzyme by weak interactions/hydrogen bonds
- They carry electrons or small groups
- Many have modified vitamins in their structure
State the function of oxidoreductases.
- Catalyse transfer of electrons from donors to acceptors
- Catalyse many oxidation-reduction reactions found in biochemical pathways
Give examples of oxidoreductases.
- Dehydrogenases accept and donate electrons as hydride ions (H:-)
- Hydrogen atoms often using coenzymes such as NAD+/NADH as an electron
donor or acceptor.
e.g. Lactate Dehydrogenase (LDH).
What is the function of transferases?
Catalyse transfer of a specific functional group between molecules
What are the four transferase enzymes and state their functions.
- Kinases - transfer phosphate groups usually from ATP to another molecule
- Aminotransferases - transfer amino groups; important in amino acid metabolism
- Glycosyltransferases - transfer carbohydrate residues
- Acyltransferases - transfer fatty acyl groups
State the function of hydrolases.
Catalyse cleavage of bonds by the addition of a water molecule
Give three example of hydrolase enzymes and their role.
- Phosphatase - hydrolyse phosphoric ester bond
- Lipase - hydrolyse ester bond in lipids
- Peptidase - hydrolyse peptide bonds to form two products
State the function of Lyase.
Catalyse the addition of water, ammonia and CO2 to double bonds or remove them to create double bonds
Give three examples of lyase enzymes and state their function.
- Decarboxylase - removes CO2
- Dehydrase - removes H2O
- Deaminase - removes NH3
State the function of isomerase.
Catalyse interconversion of isomeric forms of a molecule by transferring groups within the same molecules
Give examples of isomerase.
- Conversion of Cis and Trans molecules
- Conversion of D and L isomers
State the role of ligase.
Catalyse synthesis of new covalent bonds using ATP energy
Describe enzyme specificity.
Absolute: catalyses one type of reaction for a single substrate
Group: Catalyses one type of reaction for similar substrates
Linkage: catalyses one type of reaction for a specific type of bond
What are isoenzymes?
- Different forms of an enzyme catalysing the same reaction in different tissues
- Slight variations in the AA sequence of subunits in the quaternary structure
- Can have different physical attributes and optimal conditions
How do effector molecules change activity of an enzyme?
- By binding to the allosteric site
- Some speed up enzyme action (positive allosterism)
- Others slow enzyme action (negative allosterism)
