Enzymes Flashcards
What are enzymes
Enzymes lower the activation energy required for a chemical reaction by stabilising the transition state
RATE ENHANCEMENT
= catalysed rate/uncatalysed rate
Disease
Enzyme deficiencies caused by “inborn errors” cause disease (e.g. Phenylketonuria – lack of phenylalanine hydroxylase)
Diagnosis
Myocardial Infarction or Indigestion? - Need to be able to measure the concentration and activity of many enzymes
Drug Therapy
Many drugs are enzyme inhibitors. The specific dosage is related to the inhibition mechanism
Define the function of a lysosome
Catalyses the cutting of polysaccharide chains
Lysozyme binds to the polysaccharide chain, catalyzes the cleavage of a specific covalent bond and releases the cleaved products
What is the only valid parameter
vo (initial reaction velocity) is the only valid parameter
What is enzyme activity measured by
increasing the substrate concentration and measuring the accumulation of products over time.
What does the activity of an enzyme depend on
how rapidly it can process a substrate
What is the rate of reaction trend
The rate of reaction increases as the substrate concentration increases until a maximum is reached (Vmax)
How can Vmax be calculated
Plotting a double reciprocal of the data (Lineweaver-Burke Plot = this gives us a lot more information about the enzyme
What is Michaelis-Menten Kinetics
This is represented as Km (the constant) and is used in conjunction with Vmax to measure the activity of an enzyme using the Michaelis-Menten equation
What is the Michaelis- Menten equation
Vo = Vmax . [S] / Km + [S]
What does Km = [S] represent
The substrate concentration required for half maximum velocity
What is the biological significance of Km
Blood Glucose Levels:
Hexokinase has a lower Km and works at low concentrations of glucose. It is found in all tissues and is required for energy production in cells. A low Km ensures the utilisation of glucose even at very low concentrations
What is glucokinase
Its predominantly found in the liver and will only phosphorylate glucose when concentrations are high allowing the production of glycogen (storage).
What does a high Km ensure for glucose
it ensures its not removed from the blood for storage when the concentration is low
Where is PHOSPHO1 found
it is an enzyme found in mineralising cells such as osteoblasts and chondrocytes
Enzyme had unknown substrate/function
Amino acid sequence suggested it to be a phosphatase (enzyme which removes phosphate groups from molecules)
What are enzyme inhibitors
chemicals that interfere with enzyme reactions Many drugs are inhibitors of enzymes
What are irreversible enzyme inhibitor called
inactivators
what are reversible enzyme inhibitors called
Competitive
Allosteric (non competitive)
What is irreversible inhibition
Irreversible inhibitors react with the enzyme and form a covalent adduct with the protein.
What is competitive inhibition
A competitive drug can compete with the substrate for the active site of the enzyme.
• The substance has a similar structure to that of the normal substrate.
• The drug will occupy the active site and then leave it unchanged.
• The action of the enzyme is slowed down by the presence of a competitive inhibitor.
• A competitive inhibitor is like a key that gets though the keyhole but cannot unlock the door.
How do Competitive Inhibitors Alter the Kinetics of an Enzyme
•The inhibitor binds reversibly to the active site
•The inhibitor and substrate compete for the active site
Therefore, a higher concentration of substrate is needed to reach Vmax, resulting in an increase in Km (Vmax is unchanged)
What is allosteric inhibition
Allosteric inhibitors can bind to the enzyme at the same
time as the substrate i.e. they never bind to the active site.
• They render the enzyme and enzyme-substrate complex inactive because the inhibitor cannot be driven from the enzyme by higher substrate concentration (in contrast to competitive inhibition)
What does allosteric inhibition cause
Vmax decreases.
• Km often (but not always) increases
What are the two types of allosteric inhibition
mixed inhibition and non-competitive inhibition
Mixed inhibition
Allosteric change affects substrate binding and activity
Inhibition can be reduced but not overcome by increasing [S].
Vmax decreases due to inhibition
Km increases as substrate affinity is reduced
Non-competitive Inhibition
Allosteric change affects activity but not substrate binding
Vmax decreases due to reduced efficiency of the reaction
Km (and substrate affinity) is unchanged
Whats the function of phosphofructokinase
Catalyses transfer of phosphate from ATP to fructose 6-phosphate
Important step in glycolysis
• PFK binds ATP at two sites (active site and inhibitory site)
• In the presence of high levels of ATP, the inhibitory site is occupied and fructose 6-phosphate binding is affected
• Glycolysis does not proceed when ATP is not needed!
