Pyrroloquinoline quinone (PQQ), also called methoxatin, is a redox cofactor. It is found in soil and foods such as kiwifruit, as well as human breast milk.Enzymes containing PQQ are called quinoproteins. Glucose dehydrogenase, one of the quinoproteins, is used as a glucose sensor. PQQ stimulates growth in bacteria.

A novel aspect of PQQ is its biosynthesis in bacteria from a ribosomally translated precursor peptide, PqqA. A glutamic acid and a tyrosine in PqqA are cross-linked by the radical SAM enzyme PqqE in the first step of PqqA modification. Efforts to understand PQQ biosynthesis have contributed to broad interest in radical SAM enzymes and their ability to modify proteins, and an analogous radical SAM enzyme-dependent pathway has since been found that produces the putative electron carrier mycofactocin, using a valine and a tyrosine from the precursor peptide, MftA.

PQQ has antioxidant and B-vitamin-like activity, with a wide range of benefits for the brain and body. It promotes cognitive health and memory by combatting mitochondrial dysfunction and protecting neurons from oxidative damage. It supports energy metabolism and healthy aging.

Clinical studies in humans have shown that PQQ enhances short-term memory and attention, improves energy metabolism, and reduces markers of inflammation, as well as improving general feelings of well-being.

PQQ is known to influence multiple cellular pathways, including the production of nerve growth factor (NGF). Like CoQ10, PQQ has diverse benefits for mitochondrial function. It has been shown to attenuate mitochondrial oxidative stress as well as stimulate mitochondrial biogenesis. Mitochondrial dysfunction is a key factor in the development of numerous health conditions, especially those typically related to aging.