Enzymes Flashcards
What are enzymes
Catalysts in living cells which accelerate specific reactions in biological systems
What are catalysts
Speed up chemical reactions (lower activationenergy) while remaining chemically unchanged
Functions of enzymes
Vital for life: catalyse chemical reactions in cells that keep us alive
They have 1000s of roles including aiding:
Respiration
Digestion
Muscle and nerve function
Examples of enzymes within the oral cavity
-Alkaline Phosphatase
Found throughout body, involved in mineralisation of tissue and bone
-Maltase
Found in saliva, breaks the sugar maltose into glucose
-Amylase
Found in saliva, converts starch to sugars
-Lysozyme
Antimicrobial, breaks down peptidoglycan layer in cell wall of bacteria, found in many secretions e.g. saliva, tears, human milk, mucous
Examples of enzymes throughout the body
-Pepsin
digestive enzyme of stomach, breaks down proteins into smaller peptides
-Trypsin
found in the small intestine, breaks down proteins into amino acids
-Acetylcholinesterase
breaks down the neurotransmitter acetylcholine in nerves and muscles
-DNA polymerase
synthesises DNA from deoxyribonucleotides
Two enzyme models
Lock and key (perfect fit)
Induced fit (slightly different fit so enzyme changes shape after binding)
What enzyme is not a protein
Ribozymes (catalytic RNA molecules)
factors affecting function of enzymes
-Enzymic activity of proteins is dependent on the maintenance of their 3D structure
-Enzyme activity is affected by changes in pH and temp
-Each enzyme will usually have pH and temp optima at which is works best
-At extremes of pH and temp, the enzyme may lose its 3D structure (denature) and become totally inactive
Factors affecting the rate of enzyme reaction
Temperature
pH
Enzyme concentration
substrate concentration
Inhibitors and activators
Covalent modification
Effect of enzyme conc on reaction rate
An increase in enzyme concentration will increase the reaction rate
The equilibrium will be reached more rapidly
After equilibrium is reached the concentrations of product and substrate will not change
What is Vmax
The maximum rate of reaction
When substrate conc is smaller and in excess what does the graph look like
When substrate conc is small the rate of reaction is linearly proportional to the conc:
Therefore the rate of product formation is limited by the amount of available substrate
When the substrate conc is high the rate is nearly independent of conc:
rate of product formation now depends on the activity of the enzyme itself further increase of substrate conc will likely have little effect on rate
What is Km
The Michaelis constant, an inverse measurement of affinity
The conc of substrate at which the rate of reaction is half it’s max value V = 1/2 Vmax
What does low Km mean
Very high affinity for substrate, acts at a fairly constant rate regardless of variation in substrate concentration
RATE OF PRODUCT FORMATION IS NOT AFFECTED BY SUBSTRATE AVAILABILITY (vice versa for high Km)
What does Vi stand for
Initial rate
General symbol equation for an enzyme substrate reaction
[E] + [S] <=> [ES] => [E] + [P]
What is the Michaelis-Menten equation
V = Vmax [S] / (Km + [S])
Name the polysaccharide which is one of the main components of dental plaque
Dextran
What produces a more accurate estimate of Vmax and Km
Lineweaver-Burk plot
-Plot 1/V against 1/S
What do irreversible enzyme inhibitors do
-Damage enzymes beyond repair
-Generally cause covalent modification of the enzyme
What is the most common (natural) enzyme inhibitor
Reversible inhibitors
-Full enzyme activity is regained when the inhibitor is removed
What is feedback inhibition
When the product of a metabolic pathway builds up in the cell, inhibition of further synthesis of this product often occurs
EXAMPLE: threonine deaminase is inhibited by a build up of isoleucine
What is the function of a competitive inhibitor
Substrate and inhibitor compete for the same active binding site preventing substrates from binding once attached however this can be reversed with large quantities of the substrate
Function of Non-Competitive inhibitors
These bind to an allosteric site (not binding site) on the enzyme and alters the conformation of the enzyme so that the active site is no longer fully functional preventing the substrate from binding
Can inhibition be reversed
Competitive inhibitors are not permanent and can be overcome
What are allosteric enzymes
-Possess multiple subunits
-They possess allosteric sites to which non-substrate modulators bind changing their formation
-Subunits interact such that the binding of a substrate, inhibitor, or activator (modulators) to one subunit alters all the subunits:
-Positive modulators (increase affinity for substrate at active site)
-Negative modulators (decrease affinity for substrate at active site)
Many important regulatory enzymes are allosteric
Which enzymes do not conform to Michaelis-Menten kinetics
Allosteric enzymes
Control of enzyme activity by covalent modification
Common modification is the addition of a phosphate group to a serine, threonine or tyrosine hydroxyl group to modify an enzymes activity
