Plants are master chemists, creating amino acids from scratch. They use simple molecules like oxaloacetate and α-ketoglutarate to build complex amino acids, grouping them into families based on their origins.

Proteins, the workhorses of cells, are constantly made and broken down. Plants recycle amino acids during senescence, moving nitrogen from old leaves to growing parts. This process is key to efficient nutrient use.

Amino Acid Biosynthesis

Amino Acid Families and Biosynthetic Pathways

• Amino acids are grouped into families based on their biosynthetic pathways and precursor molecules
• Amino acid biosynthesis involves a series of enzymatic reactions that convert precursor molecules into specific amino acids
• The biosynthetic pathways for amino acids are tightly regulated to maintain optimal levels of each amino acid in the cell
• Key enzymes in amino acid biosynthesis are often subject to feedback inhibition by the end product amino acid to prevent excessive accumulation

Aspartate and Glutamate Families

• The aspartate family includes aspartate, asparagine, lysine, methionine, and threonine which are derived from aspartate as a precursor
• Aspartate is synthesized from oxaloacetate, an intermediate of the citric acid cycle, by the enzyme aspartate aminotransferase
• The glutamate family includes glutamate, glutamine, proline, and arginine which are derived from glutamate as a precursor
• Glutamate is synthesized from α-ketoglutarate, an intermediate of the citric acid cycle, by the enzyme glutamate dehydrogenase (using NADPH and NH4+)

Aromatic and Branched-Chain Amino Acids

• Aromatic amino acids include phenylalanine, tyrosine, and tryptophan which are derived from the shikimate pathway
• The shikimate pathway begins with the condensation of phosphoenolpyruvate (from glycolysis) and erythrose 4-phosphate (from the pentose phosphate pathway) to form chorismate, the precursor for aromatic amino acids
• Branched-chain amino acids include leucine, isoleucine, and valine which are derived from pyruvate and α-ketobutyrate as precursors
• The biosynthesis of branched-chain amino acids involves a series of parallel reactions catalyzed by enzymes with broad substrate specificity (can act on multiple substrates)

Protein Metabolism

Protein Synthesis and Degradation

• Protein synthesis (translation) occurs on ribosomes and involves the assembly of amino acids into polypeptide chains based on the genetic code
• The process of protein synthesis includes initiation, elongation, and termination steps which are regulated by various factors (initiation factors, elongation factors, release factors)
• Proteolysis is the breakdown of proteins into smaller peptides or individual amino acids by proteolytic enzymes (proteases)
• Proteolysis plays a crucial role in protein turnover, regulation of cellular processes, and recycling of amino acids for new protein synthesis

Nitrogen Remobilization and Senescence

• Nitrogen remobilization involves the redistribution of nitrogen from source tissues (older leaves) to sink tissues (developing leaves, fruits, seeds) during plant growth and development
• During leaf senescence, proteins are degraded and the released amino acids are transported to sink tissues for reuse in new protein synthesis
• Senescence is a highly regulated process that involves the coordinated expression of genes related to protein degradation, nutrient remobilization, and cell death
• Key enzymes involved in nitrogen remobilization during senescence include proteases, glutamine synthetase, and asparagine synthetase which convert amino acids into transportable forms (glutamine, asparagine)