Anticholinesterase Inhibition – Published Paper Review

Weinbroum, Avi A. “Pathophysiological and clinical aspects of combat anticholinesterase poisoning.” British Medical Bulletin 72, no. 1 (2004): 119-133. Accessed March 24, 2013.  http://bmb.oxfordjournals.org/content/72/

 

Anticholinesterase (AChE) is an enzyme and like all enzymes, their function is affected by several factors.

RECALL: Enzymes are affected by pH, temperature, substrate and enzyme concentration and inhibitors. These agents tend to disrupt the area of the enzyme called the active site by changing its conformation and hence hindering enzymatic activity.

INTRODUCTION

Organophosphate – like compounds are inhibitors of the enzyme acetylcholine esterase (AChE) and are referred to as nerve agents (NAs). It is related to those used in pesticides. The extremely lethal irreversibly AChE inhibitors results in an inoperative AChE causing a buildup of acetylcholine (ACh) affecting the entire cholinergic system.

DID YOU KNOW

In the 1854, Wurtz synthesized the first organophosphate compound, tetraethyl pyrophosphate.

Sad to say…. organophosphates were used as weapons of mass destruction

1980 Iraq – Iran war

1994-95 – Terrorist attacks in Japan

CLINICAL ASPECTS

The extent of absorption and contact with NA are vital issues in verifying biochemical intoxication route for the NA. NA can either be absorbed by the skin and mucous membrane or inhaled. The inhalation of the vapor results in instantaneous absorption due to the alveoli in the lungs causing respiratory ailments ranging from short breath to cardiovascular and respiratory failure and eventually death. Exposure to the skin has a nicotine-like effect causing muscle paralysis and continued developing respiratory indications and ultimately death.

BIOCHEMISTRY OF NAs

They are chemically obtained from phosphoric acids. They tend to differ slightly by substituting the -OH radical and the acid-base form but are still efficient of being both reversible and irreversible inhibitors to AChE. NAs chemical properties are such that it is both colorless and odorless in its volatile liquid state and denser that air in its gaseous state.

AChE enzyme is categorized as class three, Hydrolase and sub-class as esterase and therefore from the class it is determined that the enzyme hydrolyses esters, especially choline esters, ACh, a neurotransmitter for the cholinergic part of the nervous system.

AChE hydrolyses ACh swiftly and is located in the receptors sites of the nerves by the cholinergic nervous system. Reversible anticholinesterases are not lethal as compared to the irreversible anticholinesterases. Carbamates bind to anionic and the ester of the enzyme resulting in the separation of part of the carbamate and the formation at the location of the ester, an enzyme-cabamylated complex. Hence hydrolysis of ACh is no longer a rapid process. The most attractive binding sites for organophosphate compounds are the ester sites however the stability of the bonds depend on the correct orientation of the enzyme and inhibitor as well as other compounds are prevented from binding with the active site. They interfere with the cleft leaving the enzyme dysfunctional.

With irreversibly inhibition the NAs covalently binds (strongest bond formation) to the active site so therefore it is known as a completive inhibitor. This result in a buildup of the ACh in the neuro-effector junction hindering the synapse process in peripheral and the central cholinergic system causing toxic contamination to the nicotinic (CNS) and muscarinic cleft (muscle system). The covalent bond partakes in an instantaneous chemical reaction which stabilizes the molecule due to the surge in the thermodynamic stabilization resulting in the production of more hydrogen bridges between the phosphate and organic groups.

The skeletel muscles, the pre-ganglionic autonomic nerves and the post-ganglionic parasympathetic nerves are innervated by AChE. The cholinergic systems are bases on the muscarinic and nicotinic systems since they have receptors that display specificity to muscarinic alkaloids and nicotine alkaloids. The post-ganglionic parasympathetic fibers are innervated by the muscarinic sites which regulate the activity of the glands, smooth muscle of the respiratory, cardiovascular and gastrointestinal systems. Autonomic ganglia, part of the nicotinic sites are relied upon for the contractions of the skeletal muscle. When both muscarinic and nicotinic cholinergic neurotransmitter buildup it causes hyper-stimulation of the synaptic process and hence causes paralysis of the skeletal muscles. Comparably the buildup of AChE in the CNS nerve receptors leads to hyper-stimulation and paralysis cardiac brady-asystole, hyper-secretion from secretory glands, respirational collapse, seizures, coma and eventually it is fatal. A small window of opportunity allows antidote drugs, atropine and oximes to work against the inhibitor stimulated nicotinic and muscarinic cholinergic system correspondingly depending on the stage of poisoning.

CONCLUSION

NAs are highly poisonous complexes that result in fatality within mere seconds. The principal biochemical cause NAs is the capability to irreversible inhibit AChE enzyme resulting in the buildup of ACh in the synaptic cleavage. Antidotes such as atropine and oximes present a crucial windowed-opportunity to act against the inhibitor

 

 

Enzyme Browning Reaction

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Phenolase exist in common daily activates since they are found on a variety of fruits and vegetables that are consumed. When the fruits and vegetables such as mushrooms, potatoes and bananas are damaged it tends to turn brown over time because the enzyme phenolase present in them are exposed to oxygen and hence oxidizes the substrate found in the cells. 

Enzyme browning reaction can be controlled by several procedures such as the inactivation of the enzyme and the removal of oxygen. A process called blanching is a very effective way in controlling enzyme browning reaction by heating the fruits or vegetable to denature the enzyme before storing. In addition to heating the fruit or vegetable can be stored in ice to prevent oxidation.  Also the addition of citric acid like those found in lime or vinegar decreases the pH at which the enzyme would normally function hence inhibiting enzymatic activity.

Classes of Enzymes

Main Classes of Enzymes

1. Oxidoreductase – involved in oxidation and reduction reactions (transfers of electrons.

Example – dehydrogenase which catalyzes the removal of hydrogen and oxidase which catalyzes the addition of oxygen

 

2. Transferase – these catalyzes the transfer of groups of atoms

Example – Trans amylase catalyzes the transfer of amino groups

 

3. Hydrolases – catalyzes the addition of water or its removal from certain substrates

Example – lipases, proteases, carbohydrates, (condensation and hydration reactions).

 

4. Lyase – they break bonds by means other than hydrolysis and creates double bonds

Example – decarboxylase catalyzes the removal of carboxyl groups

 

5. Isomerase – they catalyze the transfer of groups within the molecules to yield isomeric forms

 

6. Ligases – they catalyzes the formation of C-C, C-S, C-O and C-N bonds by condensation reactions coupled to ATP cleavage.

Example – DNA ligase joins nucleotides to form DNA strands

 

Enzyme Quiz

quiztime

1. Which of the following best describes enzymes?

(A) Enzymes are biological catalysts that speed up a chemical reaction by providing and alternative pathway with lower activation energy.

(B) Enzymes are only proteins and usually it speeds up the rate of a reaction when required by the body.

(C) Enzymes are large biological molecules responsible for the thousands of chemical reactions for life sustenance.

(D) Enzymes influence chemical reactions in living systems by affecting the rate at which reactions occur

(E) Enzymes are protein catalysts that speed up a chemical reaction by lowering activation energy.

2. At high temperatures, the rate of enzyme action decreases because the increased heat

(A) changes the pH of the system

(B) alters the active site of the enzyme

(C) the reaction is in dynamic equilibrium

(D) the ratio of enzyme concentration to substrate concentration is 1:2

(E) increases the concentration of the enzyme

3. Which is true for the lock and key hypothesis?

1. substrate changes active site of the enzyme structure

2. enzyme’s active site has a complementary shape of to the substrate

3.  the functioning and role played by one enzyme is controlled by a single gene

4. R groups of the amino acids in the active site forms temporary bonds

(A) 1 only

(B) 1, 4

(C) 2, 3

(D) 2, 4

(E) All of the above

4. What defines a co-factor?

(A) organic co factors

(B) non-protein substances required by most enzymes for their efficient activity

(C) assist in the formation of the enzyme-substrate complex

(D) an unstable structure intermediate between the reactants and the products

(E)  hold substrate in a correct orientation for it to react with the active site

5. Competitive inhibition….

(A)  Binds at different sites on the enzyme not the active site

(B)   Only binds to the enzyme-substrate complex away from the active site

(C)   Binds to the active site only

(D)  Binds at a separate site from the active site

(E)   Km maybe increased or decreased

Types Reversible Inhibition

Types Reversible Inhibition

Michaelis-Menten Curve:
Effect of a Competitive/Non-Competitive Inhibitor on the Reaction Velocity vs Substrate Concentration Plot

Michaelis_Menten2

Lineweaver- Burk Plot:
Effect of a Competitive/Non-Competitive Inhibitor on the Reaction Velocity vs Substrate Concentration Plot

Lineweaver_Burk2

Once a Upon an Enzyme (part 2)

Factors Affecting the Rate of Enzyme Reactions

Temperature

Over the range 0-40 degrees Celsius the rate of reaction doubles for the rise of every 10 degrees Celsius until optimum temperature. Heat supplies activation energy and kinetic energy to the reacting molecules causing them to move more quickly thus increasing the chances of collision. Hence the in a given time the more product will form than at a lower temperature. The temperature at which enzymes catalyzes a reaction at maximum rate is called OPTIMUM TEMPERATURE. Above this temperature heat causes the molecules to vibrate so violently that the hydrophobic and ionic bond that maintain the specific 3D structures break. The enzyme molecules begin to lose its shape and activity and is said to be denatured. At first the substrate molecules loosely fits into the active site until eventually it no longer fits or can no longer be held in the correct position for the reaction to occur.

enztemprate

pH

Every enzyme functions most effectively over a narrow pH range. The optimum pH is that at which the maximum rate is achieved. A pH above or below the specific value will reduce the activity of the enzyme. Changes in pH alter the ionic charge of the acidic and basic R groups thus disrupting ionic bonds that help to maintain the specific shape of an enzyme. The pH change leads to n alteration in enzyme shape particularly at the active site thus the enzyme is unable to function effectively and is said to be denatured.

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Enzyme Concentration

If the substrate concentration is at a high level and pH and temperature are constant the rate is proportional to enzyme concentration, the larger the amount of enzyme, the greater the amount of substrate used in the time, provided its excess substrate.

Substrate Concentration

The rate increases with increasing substrate concentration up to a point where any further increases do not produce an increase in rate. As the substrate concentration increases, more and more active sites are filled so more substrate is used and more products are formed in a given time. As the concentration is increased further the active sites become saturated. Thus any extra substrate has to wait until the enzyme-substrate complex has dissociated into products and free enzyme. At high substrate concentration both enzyme concentration and the time for dissociation limit the rate of the reaction.

 

enzmichaelis

NOTE: BOTH [ENZYME] AND [SUBSTRATE] HAVE THE SAME BASIC GRAPH

for further information plz refer to Mr. J.M enzyme youtube video

Once a Upon an Enzyme (part 1)

Enzymes

 

you guys can look at this vid .. it basically sums up what is written here and is fun and kix 2 watch apart from informative

 

Enzymes are biological catalysts that speed up a chemical reaction by providing and alternative pathway with lower activation energy.

Some enzymes are RNA molecules called ribozymes.

Some antibodies have catalytic properties – abzymes.

Structure

They are globular proteins coiled into a precise 3D shape with hydrophilic R groups on the outside ensuring solubility. Most enzymes are far larger than the substrates they act on (about 100 amino acids). Only a small part of enzymes, between 3-12 amino acids comes into direct contact with the substrate. This area is called the active site and it is here that binding of the substrate takes place. The active site is always found on the cleft or crevice in the enzyme. The remaining amino acids making up the bulk of the enzyme maintain the correct globular shape of the molecule which is important if the active site is to function at an optimum rate.

Mode of Action

Enzymes act as catalysts by lowering the activation energy of the reaction. This is energy required to make substances react. Thus at a given temperature in the presence of an enzyme more reacting molecules will have energy to form product therefore more product will form in a particular time as compared to a non-catalyzed reaction.

The enzyme hold substrate in a correct orientation for them to react i.e. the active site must bind with the specific substrate. Less energy is wasted since the substrate molecules on its own will give off a lot of energy in random motion to achieve the correct orientation.

In all chemical reactions the reacting atoms or molecules pass through a transition state – an unstable structure intermediate between the reactants and the products. The amount of energy needed to get the reactants to form this transition state is the activation energy. In rearranging the bond of the reactants to form the products energy is released so the product has less energy that the reactants.

The enzyme holds the reactants properly in a precisely correct orientation to react with each other which makes bond formation and breakage easier. The formation of the enzyme-substrate complex thus lowers the activation energy for the reaction. Once the reaction has occurred the complex breaks up into enzyme and product(s) leaving the enzyme unchanged.

Lock and Key Hypothesis

This suggests that the enzyme’s active site has a complementary shape of to the substrate, into which the substrate fits exactly. Random movements of the enzyme and substrates bring the substrate into the active site forming the enzyme-substrate complex. This enzyme is like a lock into which the substrate, the key fits. The R groups of the amino acids in the active site forms temporary bonds (ionic, hydrogen and hydrophobic). This interaction can break or cause the formation of bonds in the substrate forming one or more products. When the e products form the no longer if into the active site and leave the site free to receive more substrate. Enzymes are therefor specific and will act only on one isomer. Others may act on similar molecules or break similar linkages.

lock and key

Induced Fit Hypothesis

This is when the substrate fits into the active site it causes changes in the enzyme structure such that the amino acid at the active site are molded into a precise formation allowing the enzyme to perform its catalytic function.

induced fit

Enzyme Co-factor

These are non-protein substances required by most enzymes for their efficient activity. There are three types:

  1. Inorganic ions – assist in the formation of the enzyme-substrate complex by molding the enzyme or substrate into a more suitable shape therefore increasing the chances of a reaction occurring and speeding up the rate. E.g. salivary amylase activity is increased in the presence of chloride ions. 
  2. Prosthetic groups – these are organic molecules which are tightly and permanently bound to the enzyme and act as carries of groups of atoms, single atom or electrons which are being transferred from one place to another. E.g. hem in the hemoglobin acts as an oxygen carrier.
  3. Coenzymes – these are organic molecules which are loosely attached to the enzyme and do not remain attached between reactions. They transfer groups or atoms from the active site of one enzyme to another. E.g. coenzyme A, NADP 

Properties of Enzymes

  1.  All are globular proteins
  2. They increase the rate of reaction without being used up in the reaction themselves
  3.  Their presence does not alter the nature or properties of the end product.
  4. A very small amount causes the change to large amount of substrate
  5. Their activity is affected by pH, temperature, substrate concentration, enzyme concentration and inhibitors
  6. Enzymes are substrate specific.