Biotransformation: Phase II (Conjugation) Reactions in Organic Chemistry

Biotransformation: Phase II Reactions

Introduction to Phase II (Conjugation) Reactions

Phase II reactions, also known as conjugation reactions, are essential metabolic processes in organic chemistry and pharmacology. These reactions involve the covalent attachment of small endogenous molecules to drugs or xenobiotics, increasing their hydrophilicity and facilitating excretion. Phase II reactions typically follow Phase I (functionalization) reactions and are crucial for drug metabolism and detoxification.

• Purpose: To render compounds more hydrophilic for excretion in urine or bile.

• Energy Requirement: Often require energy input, typically from high-energy phosphate bonds (e.g., ATP).

• End Products: Usually more water-soluble; exceptions include acetylation and methylation, which may increase lipophilicity.

Classification of Phase II Reactions

Main Classes of Conjugation Reactions

• Glucuronidation (conjugation with sugars)

• Sulfation (sulphate conjugation)

• Methylation

• Acetylation

• Amino Acid Conjugation

• Glutathione Conjugation

Glucuronidation

Conjugation with Sugars

Glucuronidation is the most common and important Phase II reaction, involving the conjugation of drugs or endogenous compounds with α-D-glucuronic acid. This process is catalyzed by UDP-glucuronosyltransferase and requires the cofactor UDP-glucuronic acid (UDPGA).

• Key Enzyme: UDP-glucuronosyltransferase (microsomal enzyme)

• Cofactor: UDP-glucuronic acid (UDPGA), synthesized from glucose-1-phosphate and UTP

• Substrates: Phenols, alcohols, carboxylic acids, amines, thiols, steroids, bilirubin

• Example Reaction: Glucuronidation of morphine to morphine-O-glucuronide

General Reaction:

• $\text{Drug} + \text{UDP-}\alpha\text{-glucuronic acid} \xrightarrow{\text{UDP-glucuronosyltransferase}} \text{Drug-glucuronide} + \text{UDP}$

Types of Glucuronides:

• O-glucuronides: Formed from alcohols, phenols, carboxylic acids

• N-glucuronides: Formed from amines, hydroxylamines, amides

• S-glucuronides: Formed from thiols

Clinical Relevance: Glucuronidation increases the molecular weight of metabolites, influencing their route of excretion (urinary for MW < 300, biliary for MW > 500).

Glucuronidation & Enterohepatic Circulation

Drug glucuronides excreted in bile can be hydrolyzed by gut bacteria, releasing the parent drug for reabsorption, thus prolonging drug action (enterohepatic circulation).

Drug	Glucuronidation	Excretion Route
Aspirin (MW < 300)	Yes	Urine
Rifampicin (MW > 500)	Yes	Bile

Sulfation

Sulphate Conjugation (Sulphoconjugation)

Sulfation is a major conjugation pathway for phenols and, to a lesser extent, for alcohols, amines, and thiols. It involves the transfer of a sulfate group from the high-energy donor PAPS (3'-phosphoadenosine-5'-phosphosulfate) to the substrate, catalyzed by sulfotransferase.

• Key Enzyme: Sulfotransferase (cytosolic)

• Cofactor: PAPS (energy-rich sulfate donor)

• Substrates: Phenols, alcohols, amines, thiols

• Example Reaction: Sulfate conjugation of paracetamol

General Reaction:

• $\text{Drug} + \text{PAPS} \xrightarrow{\text{sulfotransferase}} \text{Drug-sulfate} + \text{PAP}$

Methylation

Transfer of Methyl Group

Methylation involves the transfer of a methyl group to O, N, or S atoms of substrates. It is an important pathway for endogenous compounds but less so for xenobiotics. The process is catalyzed by methyltransferases and requires S-adenosylmethionine (SAM) as the methyl donor.

• Key Enzyme: Methyltransferase (cytosolic)

• Cofactor: S-adenosylmethionine (SAM), synthesized from L-methionine and ATP

• Substrates: Noradrenaline, histamine, catechols, thiols

• End Product: More lipophilic than parent compound

General Reaction:

• $\text{Substrate} + \text{SAM} \xrightarrow{\text{methyltransferase}} \text{Methylated substrate} + \text{S-adenosylhomocysteine}$

Table: Major Methyltransferases

Enzyme	Substrate	Site
Phenylethanolamine N-methyltransferase	Noradrenaline	Adrenal
Imidazole N-methyltransferase	Histamine	Liver
Catechol O-methyltransferase	Catechols	Liver, kidney, skin, nerve tissue
Hydroxyindole O-methyltransferase	N-acetylserotonin	Pineal gland
Non-specific N-methyltransferase	Various (e.g., desmethyl-imipramine)	Lung
S-methyltransferase	Thiols	Liver, kidney, lung

Acetylation

Transfer of Acetyl Group

Acetylation is common for aromatic amines, sulfonamides, and hydrazine derivatives. The reaction is catalyzed by N-acetyltransferase and uses acetyl-CoA (derived from coenzyme A) as the acetyl donor.

• Key Enzyme: N-acetyltransferase (cytosolic)

• Cofactor: Acetyl-CoA

• Substrates: Sulphathiazole, isoniazid

General Reaction:

• $\text{Drug-NH}_2 + \text{Acetyl-CoA} \xrightarrow{\text{N-acetyltransferase}} \text{Drug-NH-COCH}_3 + \text{CoA-SH}$

Amino Acid Conjugation

Conjugation with Amino Acids

Drugs with carboxylic acid groups can be conjugated with endogenous amino acids (e.g., glycine, glutamine, ornithine, arginine, taurine). This process is important for the metabolism of anti-inflammatory drugs and the formation of bile salts.

• Key Enzyme: N-acyl transferase (mitochondrial)

• Energy Requirement: ATP is required to activate the drug (not the cofactor)

• Substrates: Ibuprofen, salicylic acid, benzoic acid

General Reaction:

• $\text{R-COOH} + \text{ATP} \rightarrow \text{R-CO-AMP} + \text{PP}_i$

• $\text{R-CO-AMP} + \text{CoASH} \rightarrow \text{R-CO-SCoA} + \text{AMP}$

• $\text{R-CO-SCoA} + \text{NH}_2\text{R}'-COOH \rightarrow \text{R-CONH-R}'-COOH + \text{CoASH}$

Glutathione Conjugation

Conjugation with Glutathione

Glutathione (GSH) is a tripeptide (Gly-Cys-Glu) with a free thiol group, conferring nucleophilic properties. Glutathione conjugation is a protective mechanism for the removal of potentially toxic electrophilic compounds, especially those produced by Phase I reactions.

• Key Enzyme: Glutathione-S-transferase (cytosolic, found in liver, kidney, gut, other tissues)

• Substrates: Epoxides, haloalkanes, nitroalkanes, alkenes, aromatic halo- and nitro-compounds

• End Products: Glutathione conjugates, glycylcysteine and cysteine conjugates, mercapturic acids

General Reaction:

• $\text{Electrophilic compound} + \text{GSH} \xrightarrow{\text{glutathione-S-transferase}} \text{Glutathione conjugate}$

Mercapturic Acid Formation

• Glutamyltranspeptidase/glutathionase: Removes glutamate to form glycylcysteine conjugate

• Peptidase: Removes glycine to form cysteine conjugate

• N-acetylase: N-acetylation to yield N-acetylcysteine conjugate (mercapturic acid)

Summary Table: Phase II Conjugation Reactions

Reaction Type	Enzyme	Cofactor	Substrate	Product
Glucuronidation	UDP-glucuronosyltransferase	UDP-glucuronic acid	Phenols, alcohols, acids, amines, thiols	Drug-glucuronide
Sulfation	Sulfotransferase	PAPS	Phenols, alcohols, amines, thiols	Drug-sulfate
Methylation	Methyltransferase	SAM	O, N, S atoms	Methylated compound
Acetylation	N-acetyltransferase	Acetyl-CoA	Aromatic amines, sulfonamides	Acetylated compound
Amino Acid Conjugation	N-acyl transferase	ATP (activates drug)	Carboxylic acids	Amino acid conjugate
Glutathione Conjugation	Glutathione-S-transferase	GSH	Electrophilic compounds	Glutathione conjugate

Key Takeaways

• Phase II reactions are vital for drug metabolism and detoxification.

• Conjugation increases hydrophilicity, promoting excretion.

• Each reaction type has specific enzymes, cofactors, and substrate preferences.

• Exceptions exist (acetylation, methylation) where products may be more lipophilic.

Additional info: These reactions are central to the study of organic chemistry, especially in the context of pharmaceutical and metabolic pathways. Understanding the mechanisms and outcomes of Phase II reactions is essential for predicting drug behavior and toxicity.