Enzymes I Flashcards
What is an enzyme?
Proteins that speed up (catalyse) specific chemical reactions
Examples of the functions of an enzyme
Digestion: carbohydrates, fats, proteins
Blood clotting: fibrin clot catalysed by thrombin
Defence-immune system
Movement: muscle actomyosin is an ATPase (enzyme that breaks down ATP into ADP and inorganic phosphate)
Nerve conduction: Acetylcholinesterase breaks down acetylcholine to prevent overstimulation of cholinergic nerves
Examples of different types of enzyme and the reactions they catalyse
Proteases – Hydrolyse peptide bonds in other proteins causing proteolysis
Nucleases – Breaks the phosphodiester bond between the amino acids of nucleic acids
Polymerases – Brings about the formation of polymers, usually RNA or DNA, by bringing together the monomers of these polymers
Kinases – Able to transfer phosphate groups from phosphate-donating molecules, usually ATP, to specific substrates
Name one disease that can be caused by a defect in an enzyme
Phenylketonuria
What is Phenylketonuria?
Inherited disease caused by a mutation in the phenylalanine hydroxylase (PAH) gene
What are the consequences of having Phenylketonuria?
Low levels of the enzyme phenylalanine hydroxylase (found in the liver)
This results in the person’s body not being able to convert phenylalanine into tyrosine causing a build-up of phenylalanine to toxic levels
In high levels Phenylalanine can be broken down into toxins which can affect brain maturation
What are some examples of drugs that use enzymes as there targets? What are those enzymes?
Penicillins: Inhibit enzymes of bacterial cell wall synthesis
Anti-inflammatory agents: aspirin blocks prostaglandin biosynthesis by inhibiting the activity of the enzyme cyclooxygenase
Key features of enzymes
Increase reaction rate
Show specificity
Unchanged at end of reaction
Do not alter reaction equilibrium (allows reaction to reach equilibrium by catalysing reaction in both directions)
Facilitates reaction by decreasing the free energy of activation of the reaction
What is the free energy of activation?
The energy which must be provided to a system with potential reactants in order to produce a chemical reaction - reactant must reach transition state in order for reaction to occur
What is the transition state?
The state corresponding to the point in a reaction with the highest free energy. At this point the molecule is neither a reactant or product and is very unstable.
What is an active site of an enzyme?
3-D cavity or cleft that binds substrate(s) using electrostatic, hydrophobic, hydrogen bonding and van der Waals interactions
Where did evidence for enzyme having active sites come from?
- X-ray crystallography
2. Kinetic studies of enzyme activity
What are the 2 models of enzyme activity?
- Lock and key
2. Induced fit
How does the Lock and key model explain enzyme activity?
States that shape of active site of enzyme is completely complimentary to shape of substrate
How does the induced fit model explain enzyme activity?
States that active site of enzyme not complimentary to shape of substrate however, when substrate starts to bind enzyme undergoes conformational change which cause active site to change shape and become complimentary to shape of substrate
Which model is more accurate induced fit or lock and key?
Induced fit model more accurate
How does the activity of the enzyme Hexokinase show that the induced fit model is more accurate?
Changes shape when it forms an enzyme-substrate complex with glucose - The 2 lobes of the hexokinase squeeze out the water from the glucose
What is the enzyme-substrate binding energy?
Free energy that is released by the formation of weak interactions between a complementary substrate and enzyme
How does an enzyme reduce the free energy of activation?
Enzyme uses the enzyme-substrate binding energy to:
Bring molecules together in active site
Constrain substrate movement (brings substrates together within one space – this is called approximation)
Stabilise positive and negative charges in transition state
Provide a reaction pathway of lower energy:
To strain particular bonds in the substrate- making breakage easier.
Use cofactors (bound within active site of enzyme): bring new chemistry to active site
What is the Michaelis-menten equation?
An equation used to calculate the rate of a reaction catalysed by an enzyme
Equation: V= Vmax x (Substrate conc./substrate conc. + Km)
What is Vmax?
The rate of reaction when the enzyme is saturated with substrate
What is Km (Michaelis constant)?
Concentration of substrate needed for an enzyme to reach half its Vmax (measure of affinity of active site for its substrate)
What is the Turnover number?
The maximum amount of substrate that can be converted to product by a single active site per unit of time.
Equation: Vmax/enzyme conc.
What is the linewaver-burke equation?
Rearrangement of Michaelis-Menten equation which allows you to plot straight line graph of substrate conc. against reaction velocity
On a linewaver-burke graph what is the y-intercept equal to?
1/Vmax
On a linewaver-burke graph what is the x-intercept equal to?
-1/Km
What is competitive inhibition?
Competition for same active site on enzyme between substrate and inhibitor
Causes less enzyme – substrate complexes to form
What is the effect of competitve inhibition on Vmax and Km?
Causes Vmax to remain unchanged but does increase Km
What is non-competitive inhibition?
Substrate and inhibitor bind to different active sites on the enzyme
What is the effect of non-competitive inhibition on Km and Vmax?
Causes Vmax to decrease but Km to remain unchanged
What are the mechanisms that control enzyme activity?
Control of gene expression - Controls amount of enzyme produced by protein synthesis
Compartmentation: Signal sequence in enzyme polypeptide chain target enzyme to ER, mitochondrion, nucleus etc
Allosteric regulation: regulatory molecules
control enzyme shape - thus controlling substrate accessibility to active site
Covalent modification of enzyme - e.g. phosphorylation
What is an allosteric enzyme?
An enzyme that is able to change its shape upon binding of a ligand thus preventing access to its active sites by its substrate
What is the process of allosteric regulation?
Inhibitor (usually product of catalysed reaction) can bind to sites on regulatory peptide on allosteric enzyme
This causes a conformational change within the allosteric enzyme thus causing it to change shape
This makes the active sites of the allosteric enzyme no longer accessible to the substrate meaning enzyme can no longer catalyse reaction
Features of an allosteric enzyme
- Multisubunit complexes
- Regulatory sites and catalytic sites on different subunits
- Regulation occurs via conformational changes
- Exhibit non-Michaelis-Menten kinetics: reaction velocity vs substrate conc. plots are sigmoidal
- Involved in feedback inhibition of metabolic pathways
