The metabolic pathway of L-Arginine in fermented foods

L-Arginine (abbreviated as L-Arg), an essential amino acid for the human body, not only serves as a key nitrogen source and energy provider for microbial growth in fermented foods but also directly influences the flavor, texture, and functional properties of these foods through its metabolic processes. In the complex microecological systems of fermented foods (common microorganisms include lactic acid bacteria, yeast, acetic acid bacteria, etc.), the metabolic pathways of L-Arginine are regulated by microbial species, fermentation conditions (e.g., pH, temperature, oxygen concentration), and substrate availability. These pathways primarily revolve around two core directions: "catabolism for energy supply" and "anabolism for functional substance production," which can be specifically categorized into the following key pathways:

I. Core Catabolic Pathways: Ammonia Production, Energy Supply, and Flavor Substance Generation

In fermented foods, the primary purpose of microbial catabolism of L-Arginine is to "release nitrogen and provide energy." Meanwhile, the resulting metabolites (e.g., amines, organic acids) are crucial for shaping food flavor. The most typical pathway is the Arginine Deiminase Pathway (ADI Pathway), supplemented by the arginase pathway and oxidative deamination pathway. The metabolic directions and products of different pathways vary depending on microbial species.

1. Arginine Deiminase Pathway (ADI Pathway): Dominant Metabolic Mode of Lactic Acid Bacteria

The ADI pathway is the core metabolic route for L-ArginineL-Arg in fermented foods (especially those dominated by lactic acid bacteria, such as dairy products, fermented meat products, and fermented vegetables). This pathway operates without oxygen, serving as an "energy acquisition channel" for microorganisms in anaerobic or microaerobic environments while helping them tolerate acidic conditions (a decrease in food pH during fermentation). Its metabolic process consists of three enzyme-catalyzed steps:

Step 1: Arginine Deiminase (ADI) catalyzes the removal of the imino group from L-Arginine, producing citrulline and ammonia (NH₃). The released ammonia neutralizes part of the organic acids in the fermentation environment, alleviating acid-induced damage to microbial cells.

Step 2: Ornithine Carbamoyltransferase (OCT) further acts on citrulline, decomposing it into ornithine and carbamoyl phosphate.

Step 3: Carbamate Kinase (CK) catalyzes the reaction between carbamoyl phosphate and ADP, generating ATP, carbon dioxide (CO₂), and ammonia. ATP provides energy for microbial growth; CO₂ imparts a loose texture to fermented foods (e.g., fermented milk, pickles); and ornithine, as an intermediate product, can be further utilized by microorganisms or accumulated in the food (ornithine is a physiologically active amino acid that enhances the functional value of fermented foods).

In fermented dairy products (e.g., yogurt, kefir), lactobacilli (e.g., Lactobacillus plantarum, Lactobacillus casei) and lactococci (e.g., Lactococcus lactis) commonly metabolize L-Arginine via the ADI pathway. This not only provides energy for their proliferation but also regulates the system pH through ammonia production, delaying excessive protein coagulation. Meanwhile, accumulated citrulline and ornithine endow the products with nutritional fortification properties. In fermented meat products (e.g., salami), microorganisms such as Staphylococcus carnosus metabolize L-Arginine from raw meat via the ADI pathway; the produced ammonia inhibits the growth of spoilage bacteria, and the metabolites synergize with meat flavor substances to enrich the flavor profile of the products.

2. Arginase Pathway: Main Metabolic Direction of Yeast and Some Fungi

In foods fermented primarily by yeast (e.g., bread, wine, yellow rice wine), L-Arginine is mainly decomposed via the arginase pathway. This pathway occurs under aerobic or microaerobic conditions, with the core reaction converting L-ArginineL-Arg into urea and ornithine:

Arginase in microbial cells catalyzes the hydrolysis of L-Arginine, producing urea and ornithine.

The resulting urea is further decomposed into ammonia and CO₂ by urease. Ammonia serves as a raw material for microorganisms to synthesize other nitrogen-containing compounds (e.g., proteins, nucleic acids), while ornithine enters the "ornithine cycle" (also called the urea cycle) to maintain microbial nitrogen metabolism balance or is converted into other amino acids such as proline.

If urease activity is low in the fermentation environment (e.g., weak expression of the urease gene in some wine yeast during winemaking), urea may accumulate slightly in the food and requires further decomposition through subsequent processes (e.g., malolactic fermentation) to avoid affecting flavor (excess urea may react with ethanol to produce off-flavor substances).

In bread fermentation, Saccharomyces cerevisiae metabolizes L-Arginine from flour via the arginase pathway. The produced ammonia provides a nitrogen source for yeast to synthesize cell wall substances (e.g., chitin), ensuring yeast activity and proliferation in dough. Meanwhile, released CO₂ causes dough expansion, forming a loose bread structure. In yellow rice wine fermentation, Aspergillus oryzae and yeast synergistically metabolize L-Arginine; ornithine produced via the arginase pathway can be converted into γ-aminobutyric acid (GABA), endowing yellow rice wine with functional properties such as sedation.

3. Oxidative Deamination Pathway: Auxiliary Metabolic Mode Under Aerobic Conditions

In aerobic fermentation environments (e.g., acetic acid fermentation, some liquid fermented foods), certain microorganisms (e.g., acetic acid bacteria, some bacilli) metabolize L-Arg via the oxidative deamination pathway. The core reaction involves direct removal of the amino group from L-Arginine through oxidation, producing α-ketoarginine and ammonia, catalyzed by amino acid oxidase:

Under the action of amino acid oxidase, L-Arginine combines with oxygen to undergo oxidation, generating α-ketoarginine, hydrogen peroxide (H₂O₂), and releasing ammonia.

The resulting α-ketoarginine can be further decomposed into organic acids such as glutamic acid and succinic acid—key contributors to the sour taste of fermented foods (e.g., vinegar) that also regulate food pH and inhibit the growth of miscellaneous bacteria.

Microorganisms secrete catalase to decompose metabolically produced H₂O₂ into water and oxygen, preventing oxidative damage to cells.

In vinegar fermentation, acetic acid bacteria (e.g., Acetobacter) metabolize L-Arginine from raw materials (e.g., glutinous rice, fruits) via the oxidative deamination pathway. The produced organic acids synergize with acetic acid to form the complex sour taste of vinegar; meanwhile, ammonia release maintains nitrogen balance in the fermentation system, ensuring continuous acid production by acetic acid bacteria. In fermented soybean products (e.g., sufu), under the synergistic action of Mucor and bacteria, part of L-Arg is metabolized via this pathway; the resulting organic acids combine with esters in sufu to enhance its savory and aromatic flavor.

II. Anabolic Pathways: Microbial Self-Substance Synthesis and Functional Product Accumulation

Beyond catabolism, L-Arginine also acts as a "raw material" for microorganisms to synthesize their essential substances, participating in the production of proteins, nucleic acids, and functional small molecules. Although this process does not directly generate flavor substances, it determines microbial fermentation activity and indirectly affects the quality of fermented foods.

1. Participation in Microbial Protein and Enzyme Synthesis

L-Arginine is one of the essential amino acids for microorganisms to synthesize their own proteins. In the early stage of fermentation, when L-Arginine is abundant in the environment, microorganisms integrate it into ribosome-synthesized peptide chains to form key metabolic enzymes (e.g., deiminase in the ADI pathway, arginase in the arginase pathway), cell membrane proteins (maintaining cell structural stability), and secreted proteins (e.g., bacteriocins of lactic acid bacteria). For example, in fermented milk, Lactobacillus bulgaricus uses L-Arginine from milk to synthesize acid-producing enzymes (e.g., lactate dehydrogenase), ensuring continuous lactic acid production and promoting milk protein coagulation. In fermented vegetables, Lactobacillus plantarum synthesizes cell membrane proteins using L-Arginine, enhancing its tolerance to high-salt environments (pickle brine) and maintaining fermentation stability.

2. Synthesis of Polyamines: Regulating Microbial Activity and Food Preservation

L-Arginine is the core precursor for microbial synthesis of polyamines (e.g., putrescine, spermidine, spermine). Catalyzed by enzymes (e.g., ornithine decarboxylase, arginine decarboxylase), it is first converted into ornithine, which is then decarboxylated to form putrescine. Putrescine further reacts with carbamoyl phosphate from L-Arginine metabolism to generate spermidine and spermine. For microorganisms, polyamines are "signal molecules" that regulate cell division and delay senescence—promoting microbial proliferation in the logarithmic growth phase and enhancing stress resistance (e.g., acid tolerance, osmotic tolerance). For fermented foods, polyamines exhibit certain antibacterial effects, inhibiting the growth of spoilage bacteria (e.g., Escherichia coli, Salmonella), extending shelf life. Additionally, polyamines such as spermidine possess antioxidant properties and regulate human intestinal flora, improving the nutritional value of fermented foods. For example, in fermented meat products, spermidine synthesized by lactic acid bacteria from L-Arginine inhibits lipid oxidation and rancidity in meat, delays microbial spoilage, and maintains a tender texture. In fermented soybean products, polyamine accumulation reduces product oxidation rates and extends freshness.

3. Synthesis of Other Functional Metabolites

Under specific fermentation conditions, L-Arginine can be converted into other functional substances:

In anaerobic environments, some lactic acid bacteria convert ornithine (produced from L-Arg metabolism) into proline (an amino acid with moisturizing and antioxidant properties), endowing fermented milk and fermented grain foods with better moisture retention and anti-aging properties.

In the fermentation of yellow rice wine and beer, yeast uses L-Arginine to synthesize γ-aminobutyric acid (GABA)—a neurotransmitter with sedative and blood pressure-lowering effects, endowing fermented alcoholic beverages with functional attributes.

III. Key Factors Influencing L-Arg Metabolic Pathways

The metabolic pathways of L-Arginine in fermented foods are not fixed but regulated by multiple factors, leading to variations in metabolites and functions across different fermented foods. Core influencing factors include:

Microbial species: The primary determinant of metabolic pathways—lactic acid bacteria (e.g., lactobacilli, lactococci) mainly use the ADI pathway; yeast (e.g., Saccharomyces cerevisiae) prefers the arginase pathway; aerobic microorganisms such as acetic acid bacteria tend to use the oxidative deamination pathway. In mixed fermentation systems (e.g., yellow rice wine, sufu), multiple microorganisms act synergistically, and L-Arginine may be metabolized via multiple pathways simultaneously, forming a complex product network.

Fermentation environmental conditions:

pH affects enzyme activity (e.g., deiminase in the ADI pathway has the highest activity at pH 5.0–6.0; excessive acidity inhibits its function).

Temperature regulates L-Arginine decomposition efficiency by influencing microbial metabolic rates (e.g., lactic acid bacteria metabolize L-Arginine most rapidly at 25–37℃ during dairy fermentation).

Oxygen concentration determines metabolic direction (aerobic environments promote the arginase pathway and oxidative deamination pathway; anaerobic environments favor the ADI pathway).

Substrate concentration and ratio: The initial L-Arginine content in fermentation raw materials (e.g., high in milk and meat, low in grains) affects metabolic intensity—when abundant, microorganisms prioritize catabolism for energy; when scarce, L-Arginine is mostly used for microbial self-protein synthesis. Additionally, the presence of other amino acids in raw materials (e.g., glutamic acid, aspartic acid) competes with L-Arg for metabolic enzymes, altering metabolite proportions.

IV. Significance of Metabolic Pathways for Fermented Food Quality

The metabolic pathways of L-Arginine are not only "energy cycles" for microorganisms to maintain life activities but also directly shape the core quality of fermented foods:

Functionality: Metabolites such as ornithine, spermidine, and GABA endow fermented foods with additional values such as nutritional fortification, intestinal regulation, and antioxidation.

Flavor and texture: CO₂ ensures the loose structure of bread and fermented milk; organic acids and amines enrich the flavor profiles of vinegar and fermented meat products.

Preservation: Metabolites such as ammonia and polyamines inhibit spoilage bacteria growth, extending food shelf life.

Therefore, clarifying the metabolic pathways of L-Arginine in fermented foods provides a theoretical basis for improving food quality in a targeted manner by regulating fermentation processes (e.g., screening specific strains, optimizing temperature/pH).