Metabolism / MB - 8.1 Enzyme inhibition (HL only)

Question 1

Enzyme inhibition - understanding and basic breakdown

Answer 1

An enzyme inhibitor is a molecule that disrupts the normal reaction pathway between an enzyme and a substrate

Some chemical substances reduce/stop the activity of enzymes = inhibition

They can either be reversible (non-covalent interaction) or irreversible (strong covalent bond)

Question 2

2 types of inhibition

Answer 2

1. competitive
2. non-competitive

(Understanding = enzyme inhibitors can be competitive or non-competitive)

Question 3

Competitive inhibitors

ONLY OCCOURS WHEN THERE IS AN EXCESS OF PRODUCT
(always added in)

Answer 3

= chemicals that (compete to) bind to the ACTIVE SITE and prevent the substrate from combining with the enzyme (ie inhibitor -> active site)

The substrate and inhibitor are STRUCTURALLY/CHEMICALLY SIMILAR - when the inhibitor binds to the active site it prevents the substrate from binding

If the amount of substrate present is increased then there will eventually be more substrate collisions than inhibitor collisions.

As the inhibitor is in competition with the substrate, its effects CAN BE REDUCED by increasing substrate concentration

Question 4

Examples of competivite inhibition

Answer 4

Many antibiotics are competitive inhibtors of growth factors that are needed in bacterial metabolism

Ethanol is metabolized in the body by oxidation to acetaldehyde (which is then further oxidized to acetic acid by aldehyde ocidase enzymes - normally this process is fast ie the acetaldehyde does not accumulate in the body)

Disulfiram (anatabuse) inhibits the aldehyde oxidase which = acetaldehyde accumulation (causing sude-effects = nausea and vomiting)

This drug is sometimes used to help people overcome drinking habitions

Question 5

Examples of competivite inhibition (RELENZA

Answer 5

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 (hemagglutinin)

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

Question 6

Malonate and succinate in competivie inhibition

Answer 6

Malonate - a poison that is structurally similar to the substrate succinate

Succinate = found in the Krebs cycle of aerobic respiration and binds to the active site of the dehydrogenase enzyme.

Malonate = competes with succinate for the active site, preventing the succinate from binding = the process of respiration stops and the cell dies.

Question 7

Non-competitive inhibitors

ONLY OCCOURS WHEN THERE IS AN EXCESS OF PRODUCT

Answer 7

= do NOT bind to the active site - NOT SIMILAR TO THE SUBSTRATE

= they bind to a different site on the enzyme and CHANGE the SHAPE of the active site (the substrate may still be able to bind to the active site but the enzyme is not ably to catalyse the raction or can only do so at a slower rate)

(eg. cyanide/heavy metals (eg. lead))

Graph = never get to the same level (adding more substrate to distorted active site = cant bind = cant react = no change = never same level of production)

As the inhibitor is not in direct competition with the substrate, increasing substrate levels cannot mitigate the inhibitor’s effect (ie CAN NOT BE REDUCED)

Question 8

Examples of non-competitive inhibition

Answer 8

ARSENIC POISONING = arsenic inhibits the functioning of pyruvate dehydrogenase - an essential enzyme in a specific metabolic step reaction in cellular respiration. This results in a disrution of the energu system in cells and they die.

Question 9

Examples of non-competitive inhibition (CYANIDE)

Answer 9

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

Question 10

Control of metabolic pathways by end-product inhibition (understandings)

Answer 10

Understanding = metabolic pathways consist of chains and cycles of enzyme-catalused reactions 
Understanding = metabolic pathways can be controlled by end-product inhibition

(It is a good thing!! if there is too much it will slow, if there is not enough it will speed but BUT always allows it to GOOOOO)

Question 11

metabolic pathways

Answer 11

Metabolism describes the sum total of all reactions that occur within an organism in order to maintain life

Metabolic pathways allow for a greater level of regulation, as the chemical change is controlled by numerous intermediates

Metabolic pathways are complex with many small reactions making intermediate compounds, each controlled by its on enzyme (some are chains (eg. phenylalnine metabolism), some are cycles (eg. krebs cycles) and some are both (eg. photosynthesis))

phenylalnine metabolic pathway = shows how faulty enzymes at different points along the pathways can result in metabolic diseases such as PKU and albinism (eg. “ heal prick test”)

Question 12

End-product inhibition

Answer 12

= a form of negative feedback in which increased levels of product decrease the rate of product formation

The product binds to an allosteric site of an enzyme = a conformational change in the active site (non-competitive inhibition) as the enzyme cannot correctly function = rate of product formation will decrease

STEPS!!!
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)

This is because: metabolic enzyme pathways usually consist of chains or cycles - the product can regulate the rate of its own production by inhibiting an EARLIER ENZYME in the pathway.

(with less product, there is less enzyme inhibition)

Question 13

THREONINE to ISOLEUCINE - end-product inhibition pathway (Example)

Answer 13

Application = end-product inhibition of the pathway that convert THREONINE to ISOLEUCINE

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

Question 14

Examples of end product inhibition (more extra)

Answer 14

Regulation of ATP formation by phosphofrutokinase (enzyme in respiration) - ATP inhibitis phosphofrutokinase so when ATP levels are high, glucose is not broken down (but instead can be sotred as glycogen), when ATP levels are low, phosphofrutokinase is activated and glucose is broken down to make more ATP.

Question 15

End-product inhibition functions to ensure levels of an essential product are always tightly regulated

Answer 15

If product levels INCREASE, the product inhibits the reaction pathway and hence decreases the rate of further product formation

If product levels DECREASE, the reaction pathway will proceed unhindered and the rate of product formation will increase

Question 16

Skill: Calculating and plotting rates of reaction from raw experimental results

Answer 16

Rate of reaction (s–1) = 1 / time taken (s)

((The time taken can be measured according to either the amount of product formed or the amount of substrate consumed Reaction rate is the inverse of time taken, meaning that the reaction rate is higher when less time is taken (and vice versa)))

Question 17

Skill: Distinguishing different types of inhibition from graphs at specified substrate concentrations

Answer 17

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

Non-competitive 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 reduced

Question 18

Types of Enzymatic Reactions

Answer 18

1. exergonic

2. Endergonic

Question 19

exergonic (Enzymatic Reactions)

Answer 19

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

Question 20

Endergonic (Enzymatic Reactions)

Answer 20

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

Question 21

Extra??? - Rational Drug design

Answer 21

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