2-Amino-3-carboxy-1,4-naphthoquinone

2-amino-3-carboxy-1,4-naphthoquinone (ACNQ) is a growth stimulator for bifidobacteria that is excreted by propionic acid bacteria ^[1,2,3].

2-amino-3-carboxy-1,4-naphthoquinone is produced and excreted by Propionibacterium freundenreichii, and plays a role as an electron transfer mediator from NAD(P)H to oxygen and hydrogen peroxide in the cytoplasm of the obligate naerobe Bifidobacterium longum. B. longum does not have a respiratory chain and normally regenerates NAD(P)+ by a lactate dehydrogenase-catalyzed reaction, consuming pyruvate and excreting lactate. This process consumes an important intracellular metabolite (e.g., pyruvate), and becomes thermodynamically unfavorable if actate accumulates in the environment. 2-amino-3-carboxy-1,4-naphthoquinone offers an alternative pathway catalyzed by cytosolic diaphorase and peroxidase. By coupling the regeneration of NAD(P)+ to the reduction of O2 and H2O2, 2-amino-3-carboxy-1,4-naphthoquinone stimulates growth and is thought to protect B. longum from oxygen toxicity. The hydrophilicity of 2-amino-3-carboxy-1,4-naphthoquinone increases upon reduction, promoting its accumulation inside the cell. Therefore, only very low concentrations of extracellular ACNQ (i.e., 0.5 nM) are required for it to stimulate the growth of B. longum ^[4].

References:
[1]. Yamazaki S, Kaneko T, Taketomo N, et al. Glucose metabolism of lactic acid bacteria changed by quinone-mediated extracellular electron transfer[J]. Bioscience, biotechnology, and biochemistry, 2002, 66(10): 2100-2106.
[2]. Yamazaki S, Kano K, Ikeda T, et al. Role of 2-amino-3-carboxy-1, 4-naphthoquinone, a strong growth stimulator for bifidobacteria, as an electron transfer mediator for NAD (P)+ regeneration in Bifidobacterium longum[J]. Biochimica et Biophysica Acta (BBA)-General Subjects, 1999, 1428(2-3): 241-250.
[3]. Mori H, Sato Y, Taketomo N, et al. Isolation and structural identification of bifidogenic growth stimulator produced by Propionibacterium freudenreichii[J]. Journal of Dairy Science, 1997, 80(9): 1959-1964.
[4]. Hernandez M E, Newman D K. Extracellular electron transfer[J]. Cellular and Molecular Life Sciences CMLS, 2001, 58(11): 1562-1571.