BBT315_L-27, 28 Nitrogen metabolism

Ammonia Integration into Biomolecules

Reduced nitrogen, in the form of NH4+, is incorporated into amino acids and subsequently into other nitrogen-containing biomolecules. Glutamate and glutamine serve as crucial entry points for ammonia assimilation. Glutamate donates amino groups to most amino acids through transamination, while glutamine provides amide nitrogen for various biosynthetic processes. In numerous cells and extracellular fluids, these amino acids are present at significantly higher concentrations compared to others, sometimes exceeding by an order of magnitude.

Glutamate and Glutamine Production

Plant and Bacterial Synthesis

In plants and bacteria, glutamate is synthesized via the enzyme glutamate synthase; this pathway is absent in animals.

Animal Synthesis

In contrast, animals produce glutamate primarily through the enzyme L-glutamate dehydrogenase, highlighting a fundamental difference in nitrogen metabolism between these groups.

Glutamine Synthetase as a Regulatory Point

Glutamine synthetase plays a pivotal role in nitrogen metabolism regulation. It is composed of 12 identical subunits and is regulated through allosteric mechanisms and covalent modifications. Specific metabolites such as alanine and glycine, alongside at least six end products of glutamine metabolism, act as allosteric inhibitors. While each inhibitor induces only partial inhibition, when multiple inhibitors are present, their effects combine to nearly completely inhibit the enzyme. This regulatory mechanism enables constant adaptation of glutamine levels to meet changing metabolic demands.

Amino Acid Biosynthesis Overview

Different organisms exhibit a wide variation in their ability to synthesize essential amino acids. Most bacteria and plants can produce all 20 essential amino acids, while mammals can only manufacture about half, typically those with simpler biosynthetic pathways, termed nonessential amino acids. These nonessential amino acids, although available in healthy mammals, can also be sourced from diverse foods such as nuts, grains, meats, fruits, and vegetables, or through supplementation in cases of deficiency. Remaining amino acids, considered essential, must be acquired through dietary sources.

Essential Amino Acids

  • Arginine

  • Histidine

  • Isoleucine

  • Leucine

  • Lysine

  • Methionine

  • Phenylalanine

  • Threonine

  • Tryptophan

  • Valine

Non-Essential Amino Acids

  • Alanine

  • Asparagine

  • Aspartate

  • Cysteine

  • Glutamate

  • Glutamine

  • Glycine

  • Proline

  • Serine

  • Tyrosine

Pathway of Amino Acid Biosynthesis

All amino acids derive from intermediates in glycolysis, the citric acid cycle, or the pentose phosphate pathway. Nitrogen is introduced into these pathways via glutamate and glutamine. The complexity of biosynthetic pathways varies, with ten amino acids being just one or a few steps away from their common metabolite precursors. In contrast, biosynthetic pathways for aromatic amino acids exhibit increased complexity.

Amino Acid Biosynthetic Families

Amino acids are categorized based on their metabolic precursors, including:

  • α-ketoglutarate - Yielding glutamate, glutamine, proline, and arginine

  • 3-Phosphoglycerate - Leading to serine, glycine, and cysteine

  • Oxaloacetate - Forming aspartate and asparagine among others

Specific Amino Acid Syntheses

Serine and Glycine Biosynthesis

Serine is synthesized from 3-phosphoglycerate through various enzymatic reactions, while glycine can be derived from serine. In plants and bacteria, cysteine biosynthesis originates from serine, with the sourced reduced sulfur!

Cysteine Synthesis in Animals

In mammals, cysteine is synthesized from homocysteine and serine via cystathionine intermediates, highlighting the metabolic interconnections between sulfur and nitrogen metabolism.

Derived Molecules from Amino Acids

Many essential biomolecules stem from amino acids:

  • Glycine acts as a precursor for porphyrins, with degradation products leading to important bile pigments.

  • Creatine and phosphocreatine derive from glycine and arginine, serving as critical energy buffers.

  • Glutathione, a cellular reducing agent, is formed from three amino acids.

  • Aromatic amino acids give rise to various plant substances, while specific decarboxylation processes yield important biogenic amines, including neurotransmitters. Arginine is notably a precursor for nitric oxide, serving as a significant biological messenger.