Enzymes 2 Flashcards
Define ‘isoenzyme’
Different enzymes that catalyse the same reaction
Examples of isoenzymes include?
Glucokinase (Hexokinase IV) and Hexokinase I
How can enzymes tell us about disease?
Finding them in the wrong place can be indicative of different pathologies
Increased intracellular enzymes seen in the blood plasma is indicative of what?
Damage to cells (necrosis) due to trauma or disease
How is normal activity displayed in a clinical setting?
Samples are described in terms of arbitrary units to make it easy to spot an abnormality
Most enzyme reactions don’t follow a _____ __________.
Simple mechanism
Catalysis of a reaction with two or more substrates usually involves what?
Transfer of groups from one substrate to the other
Transfer of groups from one substrate to another can occur in what three ways?
- Sequential with ternary complex
- Non-Sequential with ternary complex
- No ternary complex
Define ‘ternary complex’
A protein complex that involves three molecules all associated with eachother. E.g. In this context, two substrates and an enzyme
An example of a sequential mechanism includes?
Assimilation of pyruvate to lactate via lactate dehydrogenase
An example of a non-sequential mechanism includes?
Formation of phosphocreatine from creatine via creatine kinase
An example of a mechanism with no ternary complex includes?
Assimilation of aspartate to form glutamate via aspartate aminotransferase
The aspartate to glutamate enzyme system is called what?
Double displacement (Ping Pong) reaction pathway
Describe and explain the kinetics of an allosteric enzyme system.
One substrate binds to a subunit causing a conformational shift of the adjacent subunit, allowing the formation of an enzyme-substrate complex more easily and so on. This results in a sigmoidal curve when plotting reaction velocity against substrate concentration
How can temperature effect enzyme activity?
Increased temperature leads to higher kinetic energy in the system, causing a higher frequency of successful collins (ES-complexes forming) - if temp. gets too high, it can denature the enzyme.
How can pH effect enzyme activity?
pH changes the charge of amino acids and thus can interfere with the 3D structure of the enzyme - ultimately leading to its denaturation
What three kinds of enzyme inhibitors are there?
- Competitive
- Non-competitive
- Uncompetitive
How do competitive inhibitors work?
They show an decreased Km and increased Vmax - the inhibitors bind more readily with the active site. Reducing the formation of enzyme-substrate complexes
What does AZT stand for?
Azidothymidine
What is AZT used to treat?
HIV
Describe the mechanism of AZT in its combating HIV
- Acts as a competitive inhibitor to reverse transcriptase
2. HIV cannot use this enzyme to produce its dsDNA from it ssRNA
How do uncompetitive inhibitors work?
- The inhibitors binds close to the active site of an enzyme
- The enzyme-substrate complex forms
- The ES complex is unable to dissociate - rendering the enzyme ineffectual
How do non-competitive inhibitors work?
The inhibitor bonds non-covalently to a part of the enzyme - the shape of the active site changes and as a result, cannot interact with substrate molecules
What happens to the Km of a system in the presence of a non-competitive inhibitor?
It doesn’t change as the substrate remains the same
What is an example of a non-competitive inhibitor?
Cyanide
CN- binds to Fe3+ cofactor of what enzyme?
Cytochrome c oxidase
In effect, what does cyanide do to the body?
Stops the production of ATP
What are the two main ways of regulating the activity of enzymes?
- Use of allosteric enzymes
2. Use of covalently-modified enzymes
An example of covalent modification in actions is _________.
Phosphorylation
Allosteric factors are usually what?
Cell metabolites that bind non-covalently to a site on the enzyme
How do these allosteric factors effect the enzyme?
They change its shape
What two models explain the kinetics of an allosteric enzyme system?
- Concerted model
2. Sequential model
Explain the concerted model of allosteric enzyme kinetics.
- Each subunit acts in the same way
- Addition of one substrate/allosteric factor ‘locks’ all others in an open position
- This makes binding of subsequent substrates much easier
Explain the sequential model of allosteric enzyme kinetics .
- The addition of a single allosteric factor/substrate causes a conformational shift in the adjacent subunit
- This shift makes it easier for a substrate to bind to the adjacent substrate
- This repeats for all subunits
How does a feedback inhibition system work?
A build up of end product of a pathway, or key junction in a pathway, can ultimately slow the entire enzymatic system
Enzymes can exist as inactive precursors called what?
Proprotein/proenzymes
How are proenzymes activated?
They can be cleaved (process know as proteolytic cleavage) by proteases
