The Crabtree effect, named after the English biochemist Herbert Grace Crabtree, describes the phenomenon whereby the yeast, ''

Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungal microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have be ...

'', produces

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

(alcohol) in aerobic conditions at high external

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

concentrations rather than producing biomass via the tricarboxylic acid (TCA) cycle, the usual process occurring aerobically in most yeasts e.g. ''

Kluyveromyces ''Kluyveromyces'' is a genus of ascomycetous yeasts in the family Saccharomycetaceae. Some of the species, such as ''Kluyveromyces marxianus, K. marxianus'', are the teleomorphs of ''Candida (genus), Candida species''. The genus name of ''Kluyve ...

'' spp. This phenomenon is observed in most species of the ''Saccharomyces'', '' Schizosaccharomyces, Debaryomyces, Brettanomyces, Torulopsis, Nematospora,'' and ''Nadsonia'' genera. Increasing concentrations of glucose accelerates glycolysis (the breakdown of glucose) which results in the production of appreciable amounts of ATP through substrate-level

phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...

. This reduces the need of oxidative phosphorylation done by the TCA cycle via the

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

and therefore decreases oxygen consumption. The phenomenon is believed to have evolved as a competition mechanism (due to the antiseptic nature of ethanol) around the time when the first fruits on Earth fell from the trees. The Crabtree effect works by repressing respiration by the

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

pathway, dependent on the substrate. Ethanol formation in Crabtree-positive yeasts under strictly aerobic conditions was firstly thought to be caused by the inability of these organisms to increase the rate of respiration above a certain value. This critical value, above which alcoholic fermentation occurs, is dependent on the strain and the culture conditions. More recent evidences demonstrated that the occurrence of alcoholic fermentation might not be primarily due to a limited respiratory capacity, but could be caused by a limit in the cellular Gibbs energy dissipation rate. For ''S. cerevisiae'' in aerobic conditions, glucose concentrations below 150 mg/L did not result in ethanol production. Above this value, ethanol was formed with rates increasing up to a glucose concentration of 1000 mg/L. Thus, above 150 mg/L glucose the organism exhibited a Crabtree effect. It was the study of tumor cells that led to the discovery of the Crabtree effect. Tumor cells have a similar metabolism, the Warburg effect, in which they favor glycolysis over the oxidative phosphorylation pathway.

References

Further reading