Part III: Nitrogen & Amino Acids
Nitrogen: The Limiting Element
Nitrogen is the most common limiting nutrient in plant growth — it constitutes approximately 1.5% of plant dry mass and is found in amino acids, nucleotides, chlorophyll, and many secondary metabolites. Plants acquire nitrogen either as nitrate (NO₃⁻) or ammonium (NH₄⁺) from soil, or — in legumes — through symbiotic nitrogen fixation by Rhizobium bacteria.
Amino acid biosynthesis in plants is markedly more complex than in animals: plants must synthesize all 20 standard amino acids de novo, including 9 that are essential for animals. The shikimate pathway, unique to plants and microorganisms, produces aromatic amino acids and is the target of the world's most widely used herbicide, glyphosate.
16 ATP
Cost of fixing N₂ to 2NH₃
~200 Tg N/yr
Global biological N fixation
7 steps
Shikimate pathway to chorismate
Chapters in Part III
Chapter 7: Amino Acid Biosynthesis
Shikimate pathway (7 steps to chorismate), aromatic amino acids (Phe, Tyr, Trp), aspartate family, glutamate family, branched-chain amino acids, glyphosate inhibition of EPSPS.
Chapter 8: Nucleotide Metabolism
Purine de novo synthesis (IMP pathway), pyrimidine de novo synthesis (UMP pathway), salvage pathways, deoxyribonucleotide synthesis, regulation.
Chapter 9: Nitrogen Fixation & Assimilation
Nitrogenase complex, N₂ + 8H⁺ + 16ATP → 2NH₃ + H₂, leghemoglobin, GS/GOGAT cycle, nitrate reduction pathway.
Key Reactions in Part III
EPSPS (glyphosate target): \(\text{Shikimate-3-P} + \text{PEP} \xrightarrow{EPSPS} \text{EPSP} + P_i\)
Nitrogenase: \(N_2 + 8H^+ + 8e^- + 16\,ATP \rightarrow 2\,NH_3 + H_2 + 16\,ADP + 16\,P_i\)
GS/GOGAT: \(\text{Glu} + NH_4^+ + ATP \xrightarrow{GS} \text{Gln}\quad\text{then}\quad\text{Gln} + \alpha\text{-KG} + NADPH \xrightarrow{GOGAT} 2\,\text{Glu}\)