• Chemical Reactions
• Transamination

 
Figure 3. Alanine transamination
Transamination is a critical reaction that allows the transfer of amino groups between amino acids and α-keto acids. The enzyme alanine transaminase (ALT) catalyzes the reversible reaction between L-alanine and α-ketoglutarate to form pyruvate and L-glutamate[1]:
L-Alanine + α-Ketoglutarate ⇌ Pyruvate + L-Glutamate
This ping-pong reaction occurs via a pyridoxal phosphate (PLP) cofactor covalently bound to the enzyme. In the first half-reaction, the amino group of L-alanine is transferred to the PLP cofactor, generating pyridoxamine phosphate (PMP) and releasing pyruvate. In the second half-reaction, the amino group is transferred from PMP to α-ketoglutarate, regenerating PLP and producing L-glutamate[2].
Transamination is crucial for shuttling nitrogen between various amino acids and for the synthesis of nonessential amino acids. The products of alanine transamination, pyruvate and glutamate, can be further metabolized in important pathways such as the citric acid cycle, gluconeogenesis, and the urea cycle[3].

• Deamination

 
Figure 4. Oxidative deamination
Oxidative deamination removes the amino group from amino acids, releasing it as ammonia (NH3). While alanine does not directly undergo oxidative deamination, the glutamate produced by alanine transamination can be oxidatively deaminated by glutamate dehydrogenase[3, 4]:
L-Glutamate + NAD+ + H2O → α-Ketoglutarate + NADH + NH4+
This reaction occurs primarily in the liver and provides a means to eliminate excess nitrogen as urea. The α-ketoglutarate formed can enter the citric acid cycle for energy production[3].
Glucose-Alanine Cycle
 
Figure 5. The glucose–alanine cycle. Alanine is synthesized in muscle by transamination of glucose-derived pyruvate, and released into the bloodstream. In the liver, the carbon skeleton of alanine is reconverted to glucose, and released into the bloodstream where it is available for uptake by muscle and resynthesis of alanine[5].  
The glucose-alanine cycle, also known as the Cahill cycle, is a metabolic pathway that allows for the transport of amino groups from skeletal muscle to the liver[6, 7]. In muscle, amino acids are catabolized for energy, with the resulting amino groups transferred to pyruvate via transamination to form alanine. Alanine then enters the bloodstream and is taken up by the liver, where it is converted back to pyruvate and glucose via gluconeogenesis. The newly synthesized glucose returns to the muscle to be used for energy production.
This cycle effectively shuttles amino groups to the liver for urea synthesis while providing a renewable source of glucose for muscle energy needs. It is particularly important during prolonged fasting or exercise when glucose levels are low and amino acid catabolism increases[2, 6].

• Peptidoglycan Synthesis

D-Alanine, the enantiomer of L-alanine, is a key component of bacterial cell wall peptidoglycan. The enzyme alanine racemase converts L-alanine to D-alanine, which is then incorporated into the peptidoglycan structure via a series of enzymatic reactions [4, 8-10].
D-Alanine first condenses with D-glutamate to form a dipeptide, which is then added to the UDP-MurNAc tripeptide precursor. The resulting UDP-MurNAc-pentapeptide is subsequently linked to the growing peptidoglycan chain. Cross-linking between peptidoglycan strands occurs through the D-alanine residues, providing structural rigidity to the cell wall [4, 8, 9].
Inhibition of D-alanine synthesis or incorporation into peptidoglycan is a common target for antibiotics, as it disrupts cell wall integrity and leads to bacterial cell lysis [8, 10].

• Alanine Dehydrogenase Reaction

 
Figure 6. alanine dehydrogenase reaction
Some microorganisms express alanine dehydrogenase (AlaDH), an enzyme that catalyzes the reversible reductive amination of pyruvate to L-alanine using NADH as a cofactor [11, 12]:
Pyruvate + NH3 + NADH + H+ ⇌ L-Alanine + NAD+ + H2O
This reaction provides an alternative route for L-alanine synthesis, particularly in organisms that lack glutamate-pyruvate transaminase. AlaDH has been purified and characterized from various bacteria and cyanobacteria, with studies revealing diversity in subunit structure, substrate specificity, and kinetic properties [11, 12].
The AlaDH reaction is of biotechnological interest for the production of L-alanine from glucose and ammonia, as it allows for a more direct synthesis route compared to the traditional transaminase-based process[11].

• Oxidation Reactions

Alanine residues in proteins can undergo oxidative modifications mediated by reactive oxygen species (ROS) such as hydroxyl radicals (•OH). These reactions typically involve hydrogen abstraction from the α-carbon, forming a carbon-centered radical that can further react with oxygen to generate peroxyl radicals and hydroperoxides[13].
Oxidative damage to alanine residues can lead to protein fragmentation, cross-linking, and functional alterations. Such modifications have been implicated in various pathological conditions associated with oxidative stress, including aging, neurodegenerative diseases, and cardiovascular disorders[13].
In addition to direct oxidation, alanine can also participate in Maillard reactions with reducing sugars, leading to the formation of advanced glycation end products (AGEs). These non-enzymatic glycation reactions are known to contribute to protein damage and inflammation in diabetes and other chronic diseases.

• Other Reactions
• Alanine engages in several other biochemical reactions, showcasing its versatility in biological systems:
• Esterification: Alanine can form esters with fatty acids or other carboxylic acids, yielding compounds such as alanine ethyl ester or N-acetylalanine.
• Peptide synthesis: As one of the 20 proteinogenic amino acids, alanine readily participates in the formation of peptides and proteins via condensation reactions catalyzed by enzymes like aminoacyl-tRNA synthetases and peptidyl transferases.
• Schiff base formation: The amino group of alanine can react with carbonyl compounds to form Schiff bases, which are important intermediates in various enzymatic reactions and in the synthesis of imines and enamines.
• Salt formation: The carboxyl and amino groups of alanine can ionize to form salts with appropriate counterions, such as sodium alanate or alanine hydrochloride

• References

1. Vroon, D.H. and Z. Israili, Aminotransferases. 2011.
2. Cole, A.S. and J.E. Eastoe, Biochemistry and oral biology. 2014: Butterworth-Heinemann.
3. Felkner, W. Nutrition Flexbook. Available from: https://courses.lumenlearning.com/suny-nutrition/.
4. Garde, S., P.K. Chodisetti, and M. Reddy, Peptidoglycan: Structure, Synthesis, and Regulation. EcoSal Plus, 2021. 9(2).
5. Ishikura, K., S.-G. Ra, and H. Ohmori, Exercise-induced changes in amino acid levels in skeletal muscle and plasma. The Journal of Physical Fitness and Sports Medicine, 2013. 2: p. 301-310.
6. Felig, P., et al., Alanine: Key Role in Gluconeogenesis. Science, 1970. 167(3920): p. 1003-1004.
7. Information, N.C.f.B. PubChem Pathway Summary for Pathway SMP0087221, Glucose-Alanine Cycle. 2024; Available from: https://pubchem.ncbi.nlm.nih.gov/pathway/PathBank:SMP0087221.
8. Barreteau, H., et al., Cytoplasmic steps of peptidoglycan biosynthesis. FEMS microbiology reviews, 2008. 32(2): p. 168-207.
9. Plapp, R. and J.L. Strominger, Biosynthesis of the peptidoglycan of bacterial cell walls: XVII. Biosynthesis of peptidoglycan and of interpeptide bridges in Lactobacillus viridescens. Journal of Biological Chemistry, 1970. 245(14): p. 3667-3674.
10. Parker, M.F.L., et al., Sensing Living Bacteria in Vivo Using d-Alanine-Derived (11)C Radiotracers. ACS Cent Sci, 2020. 6(2): p. 155-165.
11. Gu, P., et al., Alanine dehydrogenases from four different microorganisms: characterization and their application in L-alanine production. Biotechnology for Biofuels and Bioproducts, 2023. 16(1): p. 123.
12. Sawa, Y., et al., Purification and characterization of alanine dehydrogenase from a cyanobacterium, Phormidium lapideum. J Biochem, 1994. 116(5): p. 995-1000.
13. Chen, H.Y., et al., Oxygen radical-mediated oxidation reactions of an alanine peptide motif - density functional theory and transition state theory study. Chem Cent J, 2012. 6(1): p. 33.