Debaryomyces hansenii, a halotolerant and oleaginous yeast, owns a

Debaryomyces
hansenii,
a halotolerant and oleaginous yeast, owns a branched mitochondrial
respiratory chain (Breuer U et al., 2006). Besides the canonical pathway
composes by complexes I-IV, the electron flux can be driven through
alternative routes that include a cyanide-resistant alternative oxidase
(AOX), an external NADH dehydrogenase (NDH2e) and a glycerol-phosphate
dehydrogenase (MitGPDH)(Cabrera-Orefice A. et al., 2014), Fig. 1.
In yeasts, the physiological purpose
of alternative oxidoreductases is not entirely known but probably avoids
electron flux saturation on the mitochondrial cytochrome pathway
(Guerrero-Castillo et al., 2011) establishing a Radical Oxygen
Species-preventing mechanism (El-Khoury R. et al., 2013). Protection from ROS
could be explained due to this redox enzymes maintain a high oxygen
consumption without a proton pumping activity, uncoupling the oxidative
phosphorylation despite a raised transmembrane potential (Joseph-Horne et
al., 2001).
In contrast to NDH2e and MitGPDH
constitutive expression (Cabrera-Orefice et al., 2014), AOX appears at the
stationary growth phase (Veiga et al., 2003). Nevertheless, data from our
group suggest that the expression of AOX may also depend on the carbon source
added to the culture medium.
Therefore, we analysed cyanide-resistant
respiration (CRR) in intact yeast cells cultivated on different carbon
sources.
Here, we studied both cytochrome-
and alternative-pathway related oxygen consumption differences in intact
yeast cells grown on different carbon sources. In lactate- or
succinate-containing media, AOX was fully active in all growth phases. By
contrast, in glucose- or galactose-containing media, AOX activity was
detected until the late exponential phase. Since both lactate and succinate
are considered as substrates which promote respiration, their donated
electrons may saturate cytochrome route, and AOX serves as an alternative
route. However, in glycerol-containing media, AOX activity was poorly
detected in either growth phase. This result was unanticipated because glycerol
is amply used in respiratory culture media and a lactate/succinate-grown
cells behaviour will be expected. Besides, glucose-grown cells exhibited the
most significant biomass yield. Nevertheless, biomass yield in glycerol-grown
cells was higher than in the other three carbon sources. This indicates that
a considerable amount of glycerol was used in biosynthetic pathways. In this
condition, D. hansenii probably did
not express AOX to keep the Oxidative Phosphorylation system in a highly
coupled mode and support ATP needs for glycerol-promoted lipid biosynthesis.
This finding result attractive as D.
hansenii would be useful for biotechnological targets such as biodiesel
production.