8.1 Flashcards
Metabolism describes the…
sum total of all reactions that occur within an organism in order to maintain life
Most chemical changes in a cell result from a series of …
reactions (pathways), with each step controlled by a specific enzyme
- Metabolic pathways allow for a greater level of regulation, as the chemical change is controlled by numerous intermediates
Metabolic pathways are typically organised into…
chains or cycles of enzyme-catalysed reactions
Examples of chains:
Glycolysis (in cell respiration), coagulation cascade (in blood clotting)
Examples of cycles:
Krebs cycle (in cell respiration), Calvin cycle (in photosynthesis)
Every chemical reaction requires a certain amount of energy in order to proceed –
this is the activation energy (EA)
enzymes role with activation energy
- Enzymes speed up the rate of a biochemical reaction by lowering the activation energy
- When an enzyme binds to a substrate it stresses and destabilises the bonds in the substrate
- This reduces the overall energy level of the substrate’s transitionary state, meaning less energy is needed to convert it into a product and the reaction proceeds at a faster rate
Types of Enzymatic Reactions
exergonic and endergonic
exergonic
If the reactants contain more energy than the products, the free energy is released into the system (exergonic)
- These reactions are usually catabolic (breaking down), as energy is released from broken bonds within a molecule
energonic
If the reactants contain less energy than the products, free energy is lost to the system (endergonic)
- These reactions are usually anabolic (building up), as energy is required to synthesise bonds between molecules
An enzyme inhibitor is…
a molecule that disrupts the normal reaction pathway between an enzyme and a substrate
- Enzyme inhibitors can be either competitive or non-competitive depending on their mechanism of action
enzyme inhabitation prevent the formation of…
Enzyme inhibitors prevent the formation of an enzyme-substrate complex and hence prevent the formation of product
inhibition of enzymes may be either…
Inhibition of enzymes may be either reversible or irreversible depending on the specific effect of the inhibitor being used
Normal Enzyme Reaction
- In a normal reaction, a substrate binds to an enzyme (via the active site) to form an enzyme-substrate complex
- The shape and properties of the substrate and active site are complementary, resulting in enzyme-substrate specificity
- When binding occurs, the active site undergoes a conformational change to optimally interact with the substrate (induced fit)
- This conformational change destabilises chemical bonds within the substrate, lowering the activation energy
- As a consequence of enzyme interaction, the substrate is converted into product at an accelerated rate
Competitive Inhibition
- Competitive inhibition involves a molecule, other than the substrate, binding to the enzyme’s active site
- The molecule (inhibitor) is structurally and chemically similar to the substrate (hence able to bind to the active site)
- The competitive inhibitor blocks the active site and thus prevents substrate binding
- As the inhibitor is in competition with the substrate, its effects can be reduced by increasing substrate concentration
Noncompetitive Inhibition
- Non-competitive inhibition involves a molecule binding to a site other than the active site (an allosteric site)
- The binding of the inhibitor to the allosteric site causes a conformational change to the enzyme’s active site
- As a result of this change, the active site and substrate no longer share specificity, meaning the substrate cannot bind
- As the inhibitor is not in direct competition with the substrate, increasing substrate levels cannot mitigate the inhibitor’s effect
Examples of Enzyme Inhibition
Enzyme inhibitors can serve a variety of purposes, including in medicine (to treat disease) and agriculture (as pesticides)
- An example of a use for a competitive inhibitor is in the treatment of influenza via the neuraminidase inhibitor, RelenzaTM
- An example of a use for a non-competitive inhibitor is in the use of cyanide as a poison (prevents aerobic respiration)
Relenza
(Competitive Inhibitor)
- Relenza is a synthetic drug designed by Australian scientists to treat individuals infected with the influenza virus
- Virions are released from infected cells when the viral enzyme neuraminidase cleaves a docking protein (haemagglutinin)
- Relenza competitively binds to the neuraminidase active site and prevents the cleavage of the docking protein
- Consequently, virions are not released from infected cells, preventing the spread of the influenza virus
Cyanide
(Noncompetitive Inhibitor)
- Cyanide is a poison which prevents ATP production via aerobic respiration, leading to eventual death
- It binds to an allosteric site on cytochrome oxidase – a carrier molecule that forms part of the electron transport chain
- By changing the shape of the active site, cytochrome oxidase can no longer pass electrons to the final acceptor (oxygen)
- Consequently, the electron transport chain cannot continue to function and ATP is not produced via aerobic respiration
End-product inhibition (or feedback inhibition) is…
a form of negative feedback by which metabolic pathways can be controlled
In end-product inhibition..
- the final product in a series of reactions inhibits an enzyme from an earlier step in the sequence
- The product binds to an allosteric site and temporarily inactivates the enzyme (via non-competitive inhibition)
- As the enzyme can no longer function, the reaction sequence is halted and the rate of product formation is decreased
End-product inhibition functions to ensure…
levels of an essential product are always tightly regulated
- If product levels build up, the product inhibits the reaction pathway and hence decreases the rate of further product formation
- If product levels drop, the reaction pathway will proceed unhindered and the rate of product formation will increase
Isoleucine is an essential amino acid…
meaning it is not synthesised by the body in humans (and hence must be ingested)
- Food sources rich in isoleucine include eggs, seaweed, fish, cheese, chicken and lamb
In plants and bacteria, isoleucine may be synthesised from threonine in a five-step reaction pathway
- In the first step of this process, threonine is converted into an intermediate compound by an enzyme (threonine deaminase)
- Isoleucine can bind to an allosteric site on this enzyme and function as a non-competitive inhibitor
- As excess production of isoleucine inhibits further synthesis, it functions as an example of end-product inhibition
- This feedback inhibition ensures that isoleucine production does not cannibalise available stocks of threonine
The rate of reaction can be calculated according to the following formula:
Rate of reaction = 1 / time taken (s)
Factors which can influence the rate of an enzyme-catalysed reaction include
temperature, pH and substrate concentration
Competitive and non-competitive inhibitors effect the kinetics of an enzyme-catalysed reaction in different ways:
Both reduce the rate of reaction by limiting the amount of uninhibited enzyme available for reaction
Competitive Inhibitors
- Bind directly to the active site and hence exist in direct competition with the substrate
- Increasing substrate levels will increase the likelihood of the enzyme colliding with the substrate instead of the inhibitor
- The maximum rate of enzyme activity (Vmax) can still be achieved, although it requires a higher substrate concentration
Noncompetitive Inhibitors
- Bind to an allosteric site and hence do not exist in direct competition with the substrate
- Increasing substrate concentrations will not effect the level of inhibition caused by the non-competitive inhibitor
- The maximum rate of enzyme activity (Vmax) is therefore reduce
Malaria is a disease caused by parasitic protozoans of the genus Plasmodium
The life cycle of the parasite requires both a human and mosquito host – hence the disease is transmitted via mosquito bites
The maturation and development of the parasite in both human and mosquito host is coordinated by specific enzymes
By targeting these enzymes for inhibition, new anti-malarial drugs and medications can be produced
Scientists have sequenced the genome of infectious species of Plasmodium and used it to determine the parasite’s proteome
From the proteome, enzymes involved in parasitic metabolism have been identified as potential targets for inhibition
These enzymes may be screened against a bioinformatic database of chemicals to identify potential enzyme inhibitors
- Once a promising compound is identified, it may be chemically modified to improve its binding affinity and lower its toxicity
- In one particular study, over 300,000 chemicals were screened to identify 19 new chemicals that might function as inhibitors
An alternative method by which potential new anti-malarial medications can be synthesised is via rational drug design
- Rational drug design involves using computer modelling techniques to invent a compound that will function as an inhibitor
- Using combinatorial chemistry, a compound is synthesised that is complementary to the active site of the target enzyme
