Nitrogen Metabolism Overview

Nitrogen Metabolism

• Nitrogen disposal is a crucial aspect of metabolism
• The urea cycle is a key part of nitrogen metabolism
• Catabolism of carbon skeletons and nitrogen-containing substances are important elements.

Amino Acid Metabolism: Disposal of Nitrogen

• Amino acids cannot be stored; excess amino acids are catabolized
• Amino acid catabolism has two phases:
  • Phase 1: Removal of the α-amino group, forming NH4+ and α-keto acid through transamination and oxidative deamination
    • The amino group is recycled for amino acid biosynthesis or excreted in urine
    • Urea formation in the liver for excretion in urine is the major nitrogen removal route
  • Phase 2: The carbon skeleton of the α-ketoacids is converted into common metabolic intermediates (e.g., CO2, H2O, glucose, fatty acids), fueling energy production pathways

The fate of Nitrogen (N) in Different Organisms

• Ammonotelic animals: Excrete ammonia (e.g., bony fish, amphibian larvae)
• Ureotelic animals: Excrete urea (e.g., many terrestrial vertebrates, sharks)
• Uricotelic animals: Excrete uric acid (e.g., birds, reptiles).

Digestion of Dietary Proteins

• Dietary proteins are broken down into amino acids through a series of enzymatic processes in the gastrointestinal tract.
  • Enzymes like pepsin (stomach), trypsin, chymotrypsin, carboxypeptidases (pancreas), and aminopeptidases (small intestine) are involved in protein digestion
• Free amino acids and dipeptides are absorbed by epithelial cells in the small intestine and transported into the portal circulation.

Transport of Amino Acids into Cells

• Amino acids are transported into cells via active transporters.

Overall Nitrogen Metabolism

• Amino acids are the building blocks of nitrogen-containing compounds
• Amino acid catabolism is a critical process in nitrogen metabolism
• Nitrogen enters the body in various forms (from food), converted into amino acids, and then exits as urea.

Metabolic Fates of Amino Groups: Transamination and Oxidative Deamination

• The transfer of the α-amino group to α-ketoglutarate to form a-keto acids and glutamate is the first step in amino acid catabolism, known as transamination
• This reaction is catalyzed by aminotransferases.
• The process occurs in the cytosol of hepatocytes (liver cells) during transamination.
• Alternatively, oxidative deamination involves the removal of ammonia (NH4+)
• Glutamate is crucial from which the funneling of amino groups happen to, because it delivers those groups for the subsequent metabolic steps.

Substrate Specificity of Aminotransferase

• Each aminotransferase is specific to one or a few amino acids.
• α-ketoglutarate is the primary acceptor of amino groups.

Mechanism of Action of Aminotransferases

• All aminotransferases require pyridoxal phosphate (a derivative of vitamin B6), where the enzyme's catalytic activity relies on cofactor's activity.
• All amino acids except threonine and serine take part in transamination reactions.

Threonine and Serine

• These amino acids undergo dehydratase enzyme-mediated deamination.

Two Important Transferases

• Alanine aminotransferase (ALT): transfers the amino group of alanine to α-ketoglutarate to form pyruvate to glutamate, crucial for glucose-alanine cycle.
• Aspartate aminotransferase (AST): transfers the amino group of aspartate to α-ketoglutarate to form oxaloacetate and glutamate, important for urea cycle.

Oxidative Deamination

• During oxidative deamination, glutamate is deaminated in mitochondria by L-glutamate dehydrogenase to generate NH4+ and α-ketoglutarate
• The process is essential for the disposal of excess nitrogen.
• Enzymes that are involved in this process are: -L-Glutamate dehydrogenase
• Oxidative deamination is regulated by factors such as the relative concentrations of glutamate, α-ketoglutarate, and ammonia.

Transdeamination

• The combined action of aminotransferase and glutamate dehydrogenase is the process called transdeamination
• The overall reaction of amino acid catabolism involves these two reaction pathways.

Regulation of Oxidative Deamination

• The reaction's direction depends on the relative concentrations of glutamate, α-ketoglutarate, and ammonia.
• High protein intake increases glutamate and ammonia levels.
• Allosteric regulation of glutamate dehydrogenase is through ATP and GTP (inhibitors), and ADP and GDP (activators).

Oxidative Deamination: Summary

• Glutamate dehydrogenase in the mitochondrial matrix uses NAD+ or NADP+ as oxidants.
• Key outcome: liberation of ammonia and conversion of glutamate to α-ketoglutarate.
• Oxidative deamination primarily occurs in the liver and kidneys.

Glutamine Transports Ammonia

• Many extrahepatic tissues produce ammonia from metabolic processes.
• Glutamine serves as a non-toxic carrier of ammonia in the bloodstream, transporting it to the liver from other tissues.
• The amide nitrogen of glutamine is released as ammonia. Glutamine acts as the non-toxic means of delivering ammonia to the liver for more processing.

"Glucose-Alanine Cycle"

• The glucose-alanine cycle links muscle metabolism to liver metabolism, providing a means for transporting nitrogen from muscles to liver for urea synthesis.
• The cycle involves these steps:
  • Amino acid degradation in the muscles produces glutamate.
  • Glutamate forms alanine by transferring its amino group to pyruvate (formed by glycolysis).
  • Alanine travels to the liver.
  • Alanine donates its amino group to α-ketoglutarate in the liver, reforming glutamate.
  • Glutamate is deaminated to form ammonia (NH4+), which is incorporated into the urea cycle during urea synthesis.
• The cycle allows for the removal of nitrogen from muscles and the generation of glucose from pyruvate in the liver to fuel the muscles during energy demands.

The Citric Acid and Urea Cycles Are Linked

• Fumarate (a citric acid cycle intermediate) and urea cycle intermediates (e.g., aspartate) link these cycles, and are essential for each other. This allows for the disposal of ammonia while still using and/or supplying compounds from the citric acid cycle.

Urea Cycle

• Urea, the primary nitrogenous waste product of protein metabolism, is synthesized in the liver
• The urea cycle enzymes operate in both the cytosol and the mitochondria of liver cells.
• The process of urea synthesis from ammonia is referred to as the Urea Cycle.

Formation of Carbamoyl Phosphate

• Carbamoyl phosphate synthetase I, the rate limiting step, catalyzes the formation of carbamoyl phosphate.
• N-acetylglutamate plays a role as an allosteric activator.

CPS I vs. CPS II

• Carbamoyl phosphate synthetase I is essential to the urea cycle, whereas CPS II is key to the biosynthesis of pyrimidines.

Fate of Urea

• Urea diffuses from the liver and is transported to the kidneys.
• Some urea is cleaved into CO2 and NH3 in the intestine by bacterial urease.
• Remaining urea is excreted in the urine.

Metabolism of Ammonia

• Ammonia is a significant source and byproduct of nitrogen metabolism
• Several mechanisms exist for processing and removing excess ammonia from cells.

Transport of Ammonia in the Circulation

• Ammonia is continuously produced by tissues but rapidly removed to avoid toxicity
• Glutamine plays a crucial role as a non-toxic storage and transport form of ammonia, converting it to NH4+.

Catabolism of Branched-Chain Amino Acids

• The branched-chain amino acids (BCAAs) are primarily metabolized in peripheral tissues, particularly muscle.
• Degradation pathways involve transamination, oxidative decarboxylation, and subsequent degradation of the resulting α-keto acid derivatives. (e.g., products like succinyl CoA and branched chain keto acids)

Catabolism of Carbon Skeleton Amino Acids

• Amino acids can be grouped into ketogenic and glucogenic categories based on metabolic fates
• Ketogenic amino acids, such as leucine and lysine, produce ketone bodies like acetoacetate or Acetyl CoA.
• Glucogenic amino acids generate pyruvate or citric acid cycle intermediates and are used for glucose synthesis. Several amino acids have both glucogenic and ketogenic fates.

Explore the intricate processes involved in nitrogen metabolism, including the vital urea cycle and amino acid catabolism. This quiz delves into how different organisms dispose of nitrogen and the steps involved in amino acid breakdown. Test your knowledge on key metabolic pathways and nitrogen removal mechanisms.