Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 25

Distributed on the periphery of the cytoplasm, the mitochondria (mt) are spherical or rod‐shaped organelles surrounded by two membranes. The inner membrane is highly folded to form cristae. The general organization of mitochondria is the same as in higher plants and animal cells. The membranes delimit two compartments: the intermembrane space and the matrix. Mitochondria are true respiratory organelles for yeasts. Under aerobic conditions, the S. cerevisiae cell contains about 50 mitochondria. Under anaerobic conditions, these organelles degenerate, their inner surface decreases, and the cristae disappear. Supplementing the culture medium with ergosterol and unsaturated fatty acids limits the degeneration of mitochondria under anaerobic conditions. In any case, when cells formed under anaerobic conditions are placed under aerobic conditions, the mitochondria regain their normal appearance. Even in aerated grape must, the high sugar concentration represses the synthesis of respiratory enzymes. As a result, the mitochondria no longer function. This phenomenon is called catabolite repression by glucose (Section 2.3.1).

The mitochondrial membranes are rich in phospholipids—principally PC, PI, and PE (Figure 1.5). PG, a minority component in the plasma membrane, is predominant in the inner mitochondrial membrane. The fatty acids of the mitochondrial phospholipids are C16:0, C16:1, C18:0, and C18:1. Under aerobic conditions, the unsaturated residues predominate. When the cells are grown under anaerobic conditions, without lipid supplements, the short‐chain saturated residues become predominant; cardiolipin and PE diminish, whereas the proportion of PI increases. Under aerobic conditions, the temperature during the log growth phase influences the degree of unsaturation of the phospholipids—which becomes higher as the temperature decreases.

The mitochondrial membranes also contain sterols, as well as numerous proteins and enzymes (Guerin, 1991).

The two membranes, inner and outer, contain enzymes involved in the synthesis of phospholipids and sterols. The ability to synthesize significant amounts of lipids, characteristic of yeast mitochondria, is not limited by respiratory‐deficient mutations or glucose catabolite repression.

The outer membrane is permeable to most small metabolites coming from the cytosol, since it contains porin, a 29 kDa transmembrane protein possessing a large pore. Porin is present in the mitochondria of all eukaryotes as well as in the outer membrane of bacteria.

The intermembrane space contains adenylate kinase, which ensures interconversion of ATP, adenosine diphosphate (ADP), and adenosine monophosphate (AMP).

Oxidative phosphorylation takes place in the inner mitochondrial membrane, while the matrix, on the other hand, is the center of the reactions of the citric acid cycle and of the oxidation of fatty acids.

The majority of mitochondrial proteins are coded by the genes of the nucleus and are synthesized by the free polysomes of the cytoplasm. The mitochondria, however, also have their own machinery for protein synthesis. In fact, each mitochondrion possesses a circular 75 kb (kilobase pairs) molecule of double‐stranded DNA and ribosomes. The mtDNA is extremely rich in A (adenine) and T (thymine) bases. It contains a few dozen genes, which code in particular for the synthesis of certain pigments and respiratory enzymes, such as cytochrome b, and several subunits of cytochrome oxidase and of the ATP synthase complex. Some mutations affecting these genes can result in the yeast becoming resistant to certain specific mitochondrial inhibitors such as oligomycin. This property has been applied in the genetic marking of wine yeast strains. Some mitochondrial mutants are respiratory deficient and form small colonies on solid agar media. These “petite” mutants are not used in winemaking because it is impossible to produce them industrially by respiration.