Yeast Study Cracks the Code on How Cells Import Key CoA Building Blocks

Coenzyme A (CoA) underpins central metabolism across all life forms, participating in the citric acid cycle, fatty acid metabolism, and critical post-translational modifications. The textbook pathway begins with pantothenate (vitamin B5) and proceeds through five enzymatic steps requiring PANK, PPCS, PPCDC, and COASY. Mutations in any of these enzymes cause severe, often fatal neurological or cardiac diseases—including pantothenate kinase-associated neurodegeneration (PKAN)—motivating intense interest in alternative therapeutic routes.

Researchers hypothesized that pantetheine (PanSH) and 4′-phosphopantetheine (PPanSH) could serve as bypass CoA precursors, but the cellular import mechanism remained elusive. Using pantothenate-auxotrophic Saccharomyces cerevisiae strains (ecm31Δ and pan6Δ) that cannot synthesize pantothenate de novo, the team found that simply supplying PanSH or PPanSH in the medium was insufficient to support growth. A chemical co-supplementation screen revealed that adding glutathione (GSH) alongside either compound produced robust, full rescue of growth—comparable to pantothenate supplementation.

Mechanistic experiments demonstrated that the rescue depends on the spontaneous formation of mixed disulfides between PanSH or PPanSH and GSH (PanSSG or PPanSSG), confirmed by LC-MS/MS. Pre-oxidation experiments showed that 72 hours of joint incubation of PanSH and GSH were required to produce sufficient mixed disulfide and enable growth. Adding the reducing agent TCEP abolished the growth rescue, while the oxidizing agent H₂O₂ shortened the required pre-oxidation time to one hour—directly linking disulfide formation to cellular uptake. The plasma membrane transporter Opt1—previously known to import GSH, oxidized glutathione (GSSG), and other GSH conjugates—was identified as the sole transporter of (P)PanSSG. Deletion of OPT1 eliminated the growth rescue, and plasmid-based complementation restored it. Competitive inhibition by excess GSSG further confirmed Opt1 as the relevant transporter.

Once imported, genetic experiments showed that PanSH bypasses the canonical need for Cab2 (PPCS) and Cab3 (PPCDC), while PPanSH additionally bypasses Cab1 (PANK). Whole-genome sequencing of spontaneous suppressor colonies that grew on (P)PanSH without GSH revealed gain-of-function mutations in the GSH biosynthesis genes GSH1 and GSH2, confirming the necessity of endogenous GSH for natural exploitation of this route. Moreover, yeast grown on PanSH or PPanSH showed a selective growth advantage when cysteine biosynthesis was impaired—since the non-canonical route bypasses the cysteine-consuming steps of canonical CoA synthesis.

This study establishes a complete non-canonical CoA biosynthesis pathway: extracellular mixed-disulfide formation with GSH → Opt1-mediated import → intracellular conversion to CoA. The pathway is relevant to organisms that cannot efficiently use pantothenate, certain gut microbiome species, and potentially to human patients with inherited CoA pathway defects. Limitations include the use of yeast as a model organism and the absence of direct in vivo mammalian or human cell validation, leaving open questions about which mammalian transporter orthologs may serve an equivalent function.