The biosynthetic pathway of L-valine

I. Common Synthetic Framework of Prokaryotes and Eukaryotes

The biosynthesis of L-valine belongs to the metabolic network of branched-chain amino acids (BCAAs, including valine, leucine, and isoleucine). Its core pathway is highly conserved in bacteria (e.g., E. coli, Bacillus subtilis), fungi (e.g., yeast), and plants, all starting from intermediates of glycolysis or the tricarboxylic acid cycle (TCA) and generated through multiple enzymatic reactions. The entire process can be divided into three stages: precursor generation, carbon chain elongation, and amination, with key differences lying in the regulatory modes of enzymes and subcellular localization in different organisms.

II. Synthetic Pathway of Prokaryotes (Taking E. coli as an Example)

1. Generation of Precursor Substances: Two-Step Conversion of Pyruvate

First Step: Pyruvate Condensation

Two molecules of pyruvate are catalyzed by acetolactate synthase (ALS, encoded by ilvI and ilvG genes) to remove one molecule of CO₂, generating α-acetolactate. This reaction is a rate-limiting step, inhibited by feedback from L-valine (allosterically regulating the active center of ALS).

Second Step: Reductive Isomerization

α-acetolactate is acted upon by acetolactate isomeroreductase (ILVCD, encoded by ilvC and ilvD genes), consuming NADPH to generate α-hydroxyisovalerate, a branch precursor for valine and leucine synthesis.

2. Carbon Chain Modification and Amination

Dehydration and Amino Transfer

α-hydroxyisovalerate is dehydrated under the catalysis of dihydroxy acid dehydratase (ILVE, encoded by the ilvE gene) to form α-ketoisovalerate. Subsequently, α-ketoisovalerate obtains an amino group from glutamic acid under the action of branched-chain amino acid transaminase (BCAT, encoded by the ilvE gene), finally generating L-valine.

3. Regulatory Mechanisms: Feedback Inhibition and Gene Expression Regulation

Enzyme Activity Inhibition: L-valine binds to the allosteric site of ALS, reducing the enzyme's affinity for pyruvate and inhibiting precursor synthesis.

Gene Expression Regulation: In E. coli, the transcription of the ilv operon (ilvGMEDA) is regulated by BCAA-responsive transcription factors (e.g., Lrp). High concentrations of valine can inhibit operon expression through repressor proteins.

III. Synthetic Pathway and Differences in Fungi (e.g., Saccharomyces cerevisiae)

1. Functional Differentiation of Core Enzymes

Isozymes of Acetolactate Synthase: Yeast has two ALS isozymes (encoded by ILV2 and ILV6). The ILV2-type enzyme is more sensitive to valine feedback inhibition, while the ILV6-type enzyme mainly participates in leucine synthesis, synergistically regulating carbon flow distribution.

Amination Step: BCAT in yeast (encoded by BAT1 and BAT2 genes) has substrate specificity. BAT2 preferentially catalyzes α-ketoisovalerate to form valine, while BAT1 tends to synthesize leucine.

2. Subcellular Localization and Metabolic Compartmentalization

Precursor synthesis (pyruvate→α-acetolactate) occurs in the cytoplasm, while subsequent reductive isomerization and amination reactions can take place in the cytoplasm or mitochondria. This compartmentalization reduces product feedback inhibition on enzymes and improves synthesis efficiency.

IV. Synthetic Pathway and Physiological Significance in Plants

1. Exclusive Synthesis in Chloroplasts

L-valine synthesis in plants (e.g., Arabidopsis thaliana, wheat) mainly occurs in the chloroplast stroma, with precursor pyruvate derived from glycolytic intermediates in the dark reaction of photosynthesis.

The key enzyme ALS (encoded by the ILV1 gene) is inhibited by herbicides (e.g., sulfonylureas), which is the mechanism of action for such herbicides (plants die without BCAA synthesis, while animals lack this pathway).

2. Association with Secondary Metabolism

Valine can serve as a precursor for synthesizing plant stress resistance substances. For example, during injury or pathogen infection, valine can be converted into phytoalexins or volatile terpenoids (e.g., isoprene) to enhance defense capabilities.

V. Artificially Modified Pathways in Microbial Fermentation Production

1. Construction of Strains with Released Feedback Inhibition

Site-directed mutagenesis of the allosteric site of ALS (e.g., R231G mutation in ilvG of E. coli) makes it insensitive to valine feedback inhibition, thereby improving precursor synthesis efficiency.

Knockout of BCAA catabolic genes (e.g., aceE, aceF) reduces valine degradation and consumption.

2. Metabolic Flux Redistribution Strategies

Overexpression of the ilvGMEDA operon enhances the metabolic flux from pyruvate to valine.

Inhibition of competitive pathways (e.g., ilvD related to leucine synthesis) avoids carbon source shunting to other BCAAs.

VI. Key Difference Comparison of Synthetic Pathways in Different Organisms

Biological Type	Key Enzyme Regulatory Mode	Subcellular Localization	Feedback Inhibition Target
E. coli	ALS allosteric inhibition, operon transcriptional repression	Cytoplasm	ALS (ilvI/G)
Saccharomyces cerevisiae	Isoenzyme division of labor, compartmentalized metabolism	Cytoplasm/mitochondria	ILV2-type ALS
Plants (chloroplasts)	Herbicide-sensitive ALS, light-regulated expression	Chloroplast stroma	None (relying on metabolic compartmentalization)

 

VII. Evolution and Biological Significance of Synthetic Pathways

The conservativeness of the L-valine synthesis pathway reflects its basic biological function as an essential amino acid, while regulatory differences in different organisms (e.g., operon regulation in prokaryotes, compartmentalized metabolism in eukaryotes) are the result of long-term evolutionary adaptation to the environment. For example, microorganisms avoid energy consumption from excessive amino acid synthesis through strict feedback inhibition, while plants couple synthesis with photosynthesis through chloroplast localization to optimize the utilization efficiency of nitrogen and carbon sources. These mechanisms provide a theoretical basis for industrial fermentation production of valine (e.g., high-yield through genetically engineered strains).