Enzymes 2 Flashcards
What reaction do Glucokinase & hexokinase catalyze?
Glucose +ATP -> G-6-P + ADP
This traps the glucose in the cell as G-6-P
What are the kinetic properties of Gluco-&Hexokinase
Glucokinase = High Km & Vmax Hexokinase = Low Km & Vmax
Where are glucokinase & hexokinase found?
Glucokinase is in the liver
Hexokinase is in the muscles & other tissues
What is the purpose of the Glucokinase/hexokinase pair?
The hexokinase has the higher affinity so even at low glucose concs. the tissues will still have dibs on the glucose.
However when glucose conc. exceeds the hexokinase Vmax the glucokinase will step in & trap the extra in the liver
What does finding lots of intracellular enzymes in the plasma tell us?
There been cell damage due to disease or trauma
Define Isoenzymes:
Enzymes that catalyse the same reaction
How to we seperate & study plasma proteins?
Electrophoresis
In what two ways are enzymes used in diagnosis?
- comparing activity to the normal
- Examine enzymes by electrophoresis
In what two ways can an enymatic reaction occur when theres two substrates?
With or withut a ternary complex
Define an allosteric enzyme?
Allosteric enzymes are muli-subunit & contain many active sites
What is cooperative binding?
One substrate binding to a subunit causes changes in the other subunits that make further binding easie
Best example of a cooperative binding molecule?
Haemoglobin
What are the 2 types of enzyme inhibtion?
Competetive & non-competetive
How do competetive inhibitors work?
They resemble the substrate & bind to the active site.
This reduces the enzymes affinity for the substrate
Therefore Km increases
Why is Vmax unchanged when using a competetive inhibitor?
Becuase diluting the inhibitor with more substrate (increasing [S]) can overcome competetive inhibition
What effect does a competetive inhibitor have on an enzymes lineweaver-burke plot?
Increases the gradient
Why make inhibitors that mimic the transistion state over the substrate?
Enzymes have a maximum interaction with the transistion state (higher affinit bascially)
Why is it hard to produce transition state analogues?
They are difficult to isolate as theyre so temporary
How do non-competetive inhibitors wokr?
Bind to the enzyme but not the active site
Therefore Km is unchanged
However Vmax is decreased
What effect do non-competetive inhibitors have on lineweaver-burke plots?
Non-compettive inhibitors result in an increased Y-intercept (1/Vmax) and an increased gradient (Km/Vmax)
What are the 4 ways enzyme pathways are regulated?
Allosteric regulatory enzymes
Covalent modification
Feedback inhibition
Proteolytic Cleavage
Explain feedback inhibitio?
Build up of an end-product or key-junction slows the whole pathway by inhibiting an earlier enzyme
What is an allosteric effector?
A metabolite that binds to the enzyme altering its structure.
Are allosteric effectors inhibitory?
They can be both inhibitory or activators
What type of inhibitor are allostetic effectors?
Non-competetive
What are the two models of allosteric enzyme kinetics?
The concerted model
The sequential model
Explain the concerted model?
- All subunits flip between binding & non-binding conformations simultaneously
- One binding to a substrate locks them all in the ‘open’ position
- In this way low [S] has sensitised the enzyme to more S.
What shape does an allosteric enzyme M-M graph take?
A sigmoidal curve
What does the allosteric enzymes Sigmoidal curve suggesT?
Low [S] sensitizes the enzyme to better bind higher concs of [S]
How do allosteric effectors affect the concerted model?
Allosteric activators would lock a concerted model allosteric enzyme in the open position
Reverse for allosteric inhibitors
Explain the sequential model:
The subunits dont flip between ope/closed
- Open starts open
- Substrate binds to it
- Causes conformational change in next subunit (opening it)
What is the most common method of regulatory covalent modification?
Phosphorylation
What enzymes phosphorylate others?
Kinases add phosphoryl groups
Phosphotases remove them
How does phosphorylating an enzyme regulate it?
Most enzymes have multiple phosphorylation site
- Phosphorylating more sites inactivates the enzyme in increments
- This allows fine tuning of the enzymes activity
Whats a proprotein/proenzyme?
An inactive precursor to an enzyme
What enzymes conduct proteolytic cleavage?
Proteases
What is proteolytic cleavage?
Cleavage of a proenzyme to release the active enzyme
Examples of proteolytic cleavage regulation?
- Digestive enzymes
- Insulin
