Various Monascus bioactive metabolites used as food or food additives
in Asia for centuries are subjected to constant physical and chemical
changes and different Monascus genus. With the aim
to identify enzymes that participate in or indirectly regulate the
pigments and citrinin biosynthesis pathways of Monascus
purpureus cultured under high ammonium chloride, the
changes of the proteome profile were examined using sequential window
acquisition of all theoretical mass spectra–mass spectrometry-based
quantitative proteomics approach in combination with bioinformatics
analysis. A total of 292 proteins were confidently
detected and quantified in each sample, including 163 that increased
and 129 that decreased (t-tests, p ≤ 0.05).
Pathway analysis indicated that high ammonium chloride in the present
study accelerates the carbon substrate utilization and promotes the
activity of key enzymes in glycolysis and β-oxidation of fatty
acid catabolism to generate sufficient acetyl-CoA. However, the synthesis
of the monascus pigments and citrinin was not enhanced because of
inhibition of the polyketide synthase activity. All results demonstrated
that the cause of initiation of pigments and citrinin synthesis is
mainly due to the apparent inhibition of acyl and acetyl transfer
by some acyltransferase and acetyltransferase, likely malony-CoA:ACP
transacylase.
To achieve the accumulation of targeted secondary metabolites, microorganisms must adopt various protection mechanisms to avoid or reduce damage to cells caused by abiotic stresses, which formed from the changes of physical and chemical culture conditions. The protection mechanism of Monascus sp. to tolerate high-concentration ammonium chloride was analyzed by sequential window acquisition of all theoretical mass spectra−mass spectrometry proteomics in this work, and the results indicated that abiotic stresses caused by high-concentration ammonium chloride inhibited the synthesis of chitin and glycoprotein, leading to a decrease in cell wall integrity and, thus, affecting cell growth. At the same time, it also inhibited the complex enzyme III and IV activities of the mitochondrial cytochrome respiratory chain, leading to an increase in reactive oxygen species (ROS) levels. With the aim to respond to abiotic stresses, the cross-protection mechanism was implemented in Monascus, including self-protection of the Monascus cell by promoting synthesis of trehalose, a molecular chaperone that facilitates protein folding (such as heat-shock protein) and autophagy-related proteins, through not the enzyme protection system (superoxide dismutase, peroxidase, catalase, NADPH oxidase, and alternative oxidase) but the glutathione/glutaredoxin system, to maintain the intracellular redox state and then eliminate or reduce ROS damage to the cell. At the same time, an alternative respiratory pathway related to NADH dehydrogenase was activated to balance the material and energy metabolism.
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