ANTIMETABOLITES AND SUICIDE INHIBITORS

BYDEVI PRIYA SUGATHANMSC BIOCHEMISTRY AND MOLECULAR BIOLOGY

ANTIMETABOLITES

An antimetabolite is a substance that competes with, replaces or inhibits a specific metabolite of a cell and thereby interferes with the cell’s normal metabolic functioning.

It is similar in structure to a metabolite or enzymatic substrate which is normally recognized and acted upon by an enzyme to form a substance required by the cell.

Antimetabolites , because of their structural similarity to purines and pyrimidine nucleotides , they can be incorporated into either DNA or RNA and interfere with cellular function.

But they do not react in the same way with the enzyme – either the enzymatic reaction is inhibited or the antimetabolite is converted by the enzyme into an aberrant component.

Antimetabolites used for therapeutic purposes

Sulfanilamides disrupt bacterial metabolism and are used to eradicate bacterial infection is humans.

Antagonists of purines – azathioprine, mercaptopurine, thioguanine.

Antagonists of pyrimidine – fluorouracil and floxuridine.

Cytarabine which has antiviral properties interferes with dihydrofolate reductase, which is necessary for the synthesis of tetrahydrofolate and subsequently for the synthesis folic acid needed for DNA formation.

Methotrexate used in the treatment of acute leukemia, breast cancer, lung cancer and also used in low doses for the treatment of rheumatoid arthritis.

Antimetabolites as chemotherapy agents

Antimetabolite drugs were among the first effective chemotherapeutic agents discovered and are folic acid, pyrimidine or purine analogues.

They are characterized by low molecular weights.

They are similar to chemicals needed for normal biochemical activity but differ enough so that they interfere with normal cell function.

Antimetabolites induce cell death during the S phase of the cell cycle when incorporated into RNA or DNA or inhibit enzymes needed for nucleic acid production.

SUICIDE INHIBITORS Enzyme inhibition can be either reversible or irreversible.

An irreversible inhibitor dissociates very slowly from its target enzymes because it has become tightly bound to the enzyme, either covalently or non covalently.

Some irreversible inhibitors are used as important drugs.

Irreversible inhibitors can be divided into three categories: 1) Group specific

reagents 2) Substrate analogs 3) Suicide inhibitors

Suicide inhibitors or mechanism based inhibitors are modified substrates that provide the most specific means to modify an enzyme active site.

These compounds are relatively unreactive until they bind to the active site of a specific enzyme.

The inhibitors bind to the enzyme as a substrate and is initially processed by the normal catalytic mechanism.

The mechanism of catalysis then generates a chemically reactive intermediate that inactivates the enzyme through covalent modification.

The fact that the enzyme participates in its own irreversible inhibition, strongly suggests that the covalently modified group on the enzyme is catalytically vital.

These compounds are also called mechanism based inhibitors because they hijack the normal enzyme reaction to inactivate the enzyme.

The process is known as suicide inhibition also known as suicide inactivation or mechanism based inhibition – a type of irreversible form of enzyme inhibition.

N,N- dimethylpropargylamine as suicide inhibitor

o A flavin prosthetic group of monoamine oxidase (MAO) oxidizes the N,N- dimethylpropargylamine, which in turn inactivates the enzyme by covalently modifying the flavin prosthetic group by alkylating N-5.

o Monoamine oxidase deaminates neurotransmitters such as dopamine and serotonin, lowering their levels in the brain.

o Parkinson disease is associated with low levels of dopamine, and depression is associated with low levels of serotonin.

o The drug (-)deprenyl, which is used to treat Parkinson disease and depression, is a suicide inhibitor of monoamine oxidase.

MEDICAL IMPORTANCE Suicide inhibitors play a significant role in rational drug design, a

modern approach to obtaining new pharmaceutical agents in which chemists synthesize novel substrates based on knowledge of substrates and reaction mechanisms.

Drugs are unreactive until within the enzymes active site and is highly specific, so it has an advantage very few resulting side effects.

Some clinical examples of suicide inhibitors include : Aspirin Penicillin Clavulanic

acid Sulbactam Allopurinol Eflornithine Sarin 5-

Fluorouracil

Aspirin which inhibits cyclooxygenase 1 and 2 enzymes.

Penicillin is a suicide inhibitor. Glycopeptide transpeptidase catalyzes the formation of cross links between D- amino acids in the cell walls of bacteria. This enzyme also catalyzes the reverse reaction , the hydrolysis of peptide bonds. During the course of hydrolyzing the strained peptide bond in penicillin, the enzyme activates the inhibitor, which then covalently modifies an active site serine in the enzyme. In effect, the enzyme “commits suicide” by hydrolyzing the strained peptide bond in penicillin.

PENICILLIN RESISTANCE

Human use of penicillin and its derivatives has led to the evolution of strains of pathogenic bacteria that express 'β- lactamases , enzymes that cleave β- lactam antibiotics , rendering them inactive.

Human medicine responded with the development of compounds such as clavulanic acid, a suicide inhibitor , which irreversibly inactivates the β- lactamases .

Clavulanic acid mimics the structure of a β- lactam antibiotic and forms a covalent adduct with a Ser in the β- lactamase active site.

This leads to a rearrangement that creates a much more reactive derivative, which is subsequently attacked by another nucleophile in the active site to irreversibly acylate the enzyme and inactivate it.

Amoxicillin and clavulanic acid are combined in a widely used in a pharmaceutical formulation with the trade name Augmentin.

Strains of disease causing bacteria that are resistant to both amoxicillin and clavulanic acid have been discovered, and now we have to develop new antibiotics.

Allopurinol which inhibits uric acid production by xanthine oxidase in the treatment of gout.

Eflornithine, one of the drugs used to treat sleeping sickness, is a suicide inhibitor of ornithine decarboxylase.

Sarin is a suicide inhibitor of acetylcholinesterase.

5- Fluorouracil acts as a suicide inhibitor of thymidylate synthase during the synthesis of thymidine from uridine.