Lecture 7.1: Enzymes Flashcards
What is rate of reaction influenced by? (4)
• Concentration of reactants
• Activation energy
• Temperature
• Presence of a catalyst
Activation Energy
For a reaction to occur, the reactants must acquire enough energy to enable
them to react with each other
This energy comes from e.g. reactants colliding with each other
The greater the activation energy, the slower the rate of reaction
Catalysts
They reduce Ea needed for a reaction to occur
They may bring reactants closer together and in an appropriate orientation for the reaction to proceed
A compound that speeds up a chemical reaction without itself undergoing any permanent change, so is free to be used again
What is an enzyme?
It is a biological catalyst – made from proteins each has a unique amino
acid composition each has a unique 3D structure
Mechanisms of Enzyme Action? (2)
1) Lock and key
2) Induced fit
Why are enzymes selective?
Because they have unique 3D structures
This means they display selectivity for the reactions they catalyse
The substrate(s) they work with must fit the active site (analogous to ligand bind region on receptors)
Absolute Enzymes
Only catalyse one specific reaction
Group Enzymes
Only act on molecules with a specific functional group (e.g. methyl groups)
Linkage Enzymes
Will only act on a specific type of chemical bond (e.g. peptide bonds)
Stereochemical Enzymes
Will only act on a particular optical isomer of a molecule
Classification of Enzymes (6)
Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases
Oxidoreductases Reaction Type
Oxidation-Reduction
Transferases Reaction Type
Transfer of groups e.g. amino, carboxyl between molecules
Hydrolases Reaction Type
Cleave bonds coupled with the insertion of water
Lyases Reaction Type
Cleavage of C-C, C-S and C-N bonds
Isomerases Reaction Type
Bond rearrangement
Ligases Reaction Type
Bond formation between C-O, C-N and C-S
What can rate of enzyme catalysed reactions can be influenced by? (7)
Substrate concentration
Enzyme concentration (in real life, [S] are generally»_space; [E])
Enzyme activity (influence of other factors e.g. allosteric modulators that may alter properties of the active site e.g. increasing affinity for substrate)
pH
Temperature
Presence of ions
Presence of inhibitors
The significance of Km values: low Km and high Km
Km values represent affinity of an enzyme for its substrate
Low Km = high affinity for substrate
High Km = low affinity for substrate
What is 1 unit on a Vmax/V0 graph
1 unit = the amount of enzyme that converts 1micromol of product per min under standard conditions
What is the standardised rate?
Per litre (L) of serum or per gram (g) of tissue
Enzyme Inhibitor Types (2 + 2 sub)
Irreversible
Reversible [Competitive and Non-Competitive]
Irreversible Enzyme Inhibitor
Bind very tightly, generally form covalent bond(s)
Examples: nerve gases, sarin blocks acetylcholine esterase
Competitive Enzyme Inhibitor
Binds at active site
Affects Km, not Vmax
Non-Competitive Enzyme Inhibitor
Binds at another site on the enzyme
Affects Vmax but not Km
What is Km?
The concentration of substrate which permits the enzyme to achieve half Vmax
What is Vmax?
The reaction rate when the enzyme is fully saturated by substrate
Competitive Inhibition Mechanism
• Resembles the substrate
• Binds to the active site
• Reduces the proportion of enzyme molecules
bound to the substrate
Non-Competitive Inhibition Mechanism
• Binds at an alternative site on enzyme (allosteric
site)
• Alters the shape of the molecule (enzyme)
• Decreases the turnover number of the enzyme→
lowering Vmax
What is the Turnover Number?
The turnover number is the number of substrate molecules transformed per minute by a single enzyme molecule
Long Term Regulation of Enzyme Activity
Change in rate of protein synthesis
• Enzyme induction/repression
• Through a change in transcription/translation
Change in rate of protein degradation
• Ubiquitin-proteasome pathway
Short Term Regulation of Enzyme Activity (seconds to hours)
Substrate and product concentration
Change in enzyme conformation:
• Allosteric regulation
• Covalent modification
• Proteolytic cleavage (irreversible)
Feedback Inhibition
The end product of a metabolic pathway has the ability to inhibit a enzymatic step upstream
Allosteric Enzymes
• Distinct regulatory sites
• Regulation by small signal molecules
• Multiple active sites (multi-subunit)
• Binding at regulatory site induces
conformational change that affects active site
• Activity at one active site affects other active
sites, known as co-operativity
Allosteric Regulation: T vs R & Co-operativity
Tense & Relaxed:
• T state – low affinity
• R state – high affinity
Co-operativity:
• Substrate binding to one subunit favours others becoming relaxed
Allosteric Activators
Increase the proportion of enzyme in the R state
Shift curve to the left
Allosteric Inhibitors
Increase the proportion of enzyme in the T state
Shift curve to the right
Allosteric regulation of phosphofructokinase
Phosphofructokinase is allosterically regulated and sets the pace of glycolysis
It is the key regulator of glycolysis
Allosteric activators: AMP, fructose-2,6-bisphosphate
Allosteric inhibitors: ATP, citrate, H+
Covalent Modification - Phosphorylation
Protein kinases transfer the terminal phosphate from ATP to the –OH hydroxyl group of Serine, Threonine, Tyrosine
Covalent Modification - Dephosphorylation
Protein phosphatases reverse the effects of kinases
They do this by catalysing the hydrolytic removal of phosphoryl groups from proteins
Clinical Significance of Enzymes (2)
Drug Targets
Biomarkers
Clinical Significance of Enzymes: Drug Targets
Angiotensin converting enzyme inhibitors (e.g. Enalapril) – mainstay of the treatment of hypertension
Inhibitors of dihydrofolate reductase (e.g. methotrexate) causes cells to arrest in S phase of cell cycle and die – some cancers and rheumatoid arthritis
Clinical Significance of Enzymes: Biomarkers
Liver function tests measure a range of hepatic enzymes along with plasma
levels of other substances
Creatine kinase is a marker of muscle damage
