Enzymes 2 Flashcards
Give an example of two enzymes that catalyse the same reaction?
Glucokinase and hexokinase
What reaction to glucokinase and hexokinase catalyse?
Glucose + ATP → Glucose-6-phosphate + ADP
Where is glucokinase found?
In the liver
Where is hexokinase found?
Everywhere other than the liver
What kinetic properties do glucokinase and hexokinase have (KM and vmax)
What does a graph of the activity of glucokinase and hexokinase look like and explain it?
Hexokinase is constantly working flat out whereas glucokinase can respond proportionally to blood glucose
Why does hexokinase have a low KM?
So it can respond proportionally to blood glucose
When [glucose] is low you don’t want the liver to break it down as you want it to be released into the blood
What is the difference between glucose-6-phosphate and glucose?
G-6-P cannot leave the cell
What do enzymes being found where they shouldn’t be indicate?
Tissue damage, enzymes may escape from damaged cells
How is enzyme activity measured in clinical scenarioes?
With an arbitory value
1U/ml o 100% = normal
What is an isozyme?
Different enzymes that catalyse the same reaction
Where are isoenzymes often found relative to each other?
In different tissues
Are isoenzymes coded for by the same gene?
No, they are products of different genes
What is electrophoresis?
Process where you can seperate out biological molecules
How does electrophoresis work?
Larger molecules get stuck and so move less than smaller molecules, charge also has an impact
What is creatine kinase (CK)?
An example of an isoenzyme which is a dimer made up from two polypeptide chains B and M
What is a dimer?
Oligomer consisting of two monomers joined by bonds that can be strong or weak, covalent or intermolecular
What is an oligomer?
Molecule complex of chemicals that consists of a few monomer units
What are the 3 isomorms that creatine kinase (CK) can form?
CK1 (BB)
CK2 (BM)
CK3 (MM)
How are enzymes used in diagnosis?
Measure activity and compare with normal
Sepperate differnt forms of enzymes by electrophoresis
Are enzymes restricted to interacting with one substance?
No, most enzymes can interact with a variety of substances
How does the KM vary between enzymes and their different substrates?
Changes for each substrate
What does catalysing a reaction with two or more substances normally involve?
The transfer of groups from one to the other
What ways can the catalyse of two or more substances with the transfer of groups occur?
Random order or ordered with ternary complex
No ternary complex formation
What does a random order reaction involving a ternary complex look like?
What does an ordered reaction involving a ternary complex look like?
With does a reaction with no formation of a ternary complex look like?
What is an example of an enzyme that uses an ordered sequential mechanism?
Lactate dehydrogenase for the reaction of pyruvate to lactate
What is an example of an enzyme that uses a random sequential mechanism?
Creatine kinase (CK) for creatine to phosphocreatine
What do ordered and unordered squential mechanisms have in common?
The ternary complex is formed first
What is an example of an enzyme that uses a no formation of tertary complex mechanism?
Asparate aminotransferase, uses a double displacement reaction
What is a double displacement reaction?
Substrates bounce on and off the enzyme
What happens when you take an amino group of an amino acid?
It becomes a ketoacid
What notation is commonly used to show the mechanisms of enzyme reactions?
Clelend notation
What is an allosteric enzyme?
Enzymes made up of many subunits
What sites are on an allosteric enzyme?
Can contain many active sites, they have other sites that can be used for regulation
What do the kinetics of an allosteric enzyme look like and why is this?
Because of cooperative binding
What is cooperative binding?
One substance binds to an enzyme subunit causing a change in the acitve site of other sub units
What is an example of an enzyme that uses cooperative binding?
Haemoglobin
What factors affect the way that enzymes function?
Temperature
pH
Inhibition
How does temperature affect enzyme functionality?
As temperature increases, molecule collisions increase
As temperature increases, internal energy of molecule increases
If temperature increases further the enzyme will denature
How does pH affect enzyme functionality?
Changes charge of amino acids
If the active amino acids change charge will stop working
Extreme pH will denature the enzyme
Also affects substrates
What are the three types of inhibitors?
Competative inhibitor
Non-competative inhibitor
Uncompetative inhibitor
What is a competative inhibitor?
Binds to the active site non covalently, competing with the substrate
What do competative enzymes cause in terms of enzyme kinetics?
Decreased affinity (KM increase)
vmax remains the same as increasing [substrate] can overcome it
What does the best inhibitor look like and why?
The transition state because it binds better
Why are there not a lot of transition state drugs?
The transition state is hard to determine
What are non-competative inhibitors?
Bind to somewhere other than the active site non covalently
What do non-competative inhibitors cause in terms of enzyme kinetics?
KM unchanged as substrate can still bind
vmax decreases because cannot out compete the inhibitor
What do inhibitors on a Lineweaver-Burk plot look like?
What is an example of an irreversible inhibitor?
Cyanide (CN-)
What does biological reactions occuring in pathways allow?
Regulation of a key step byt regulating the enzyme, often the first step
What are the 2 main ways of regulating enzymes?
Allosteric enzymes
Covalently modified enzymes
What is feedback inhibition?
Build up of an end product inhibits an earlier enzyme
What kind of inhibition do pathways often use?
Feedback inhibition
What are allosteric effectors?
Cell metabolites that bind non covalently to a site on the enzyme that is not the active site
What do effectors do to an enzyme structure?
Cause it to change
What are the two types of effectors?
Inhibitor
Activator
What two models explain allosteric kinetics?
Concerted model
Sequential model
What does the concerted model state?
Sub units exist in two conformations (one binds well and the other doesn’t)
With no substance the enzyme flips between the two conformations
All sub units must be in the same conformation (flip in concert)
What are allosteric activators?
Bind somewhere other than the active site, locking the open conformation
What are allosteric inhibitors?
Bind somewhere other than the active site, locking in the closed conformation
What are the properties of the sequential model?
Substrate binding causes a change in one subunit
Binding causes conformational change
What is an example of a reversible covalent modificaiton that can inhibit an enzyme?
Being phosphorylated
What are the two kinds of enzymes that catalyse the phosphorylation of an enzyme?
Protein kinases (adds phosphory group to a protein)
Protein phophotases (removes phosphoryl group)
What do multiple phosphorylation sites allow?
Fine control of an enzymes function, it is never completely off or on but is finely tuned
What is a proprotein?
Enzymes existing as an inactive precurser protein
What happens when the proprotein is cleaved of an enzyme?
Enzyme becomes active by proteases
What kinds of enzymes often use proprotein for regulation?
Digestive enzymes
What is proteolysis?
Process of breaking peptide bonds between amino acids
What is a protease?
Enzymes that perform proeolysis (protein catabolism by hydrolysis of peptide bonds)
