Periplasmic space yeast

Many of the E. coli psi genes function to enhance Pi availability in, and uptake from, the external medium. For example, phosphate starvation induces pho A whose product is alkaline phosphatase, a hydrolytic enzyme that is excreted into the periplasmic space where it acts to cleave extracellular organic P to Pi. A second psi gene system, the phosphate-specific transport (Pst) operon uses energy to transport Pi across the E. coli membrane. The affinity of this four-gene transport system is much greater than that of the constitutive Pi shuttle. Many of these same molecular starvation rescue mechanisms have been characterised in yeast. 

Fungi present a much more complex target than bacteria, eg yeasts, which constitute a large group of rather disparate eukaryotic organisms, are enveloped by a mixture of glucan, mannan, chitin, and lipoproteins, and separated from the plasma membrane by a periplasmic space. 

Fig. 2 Ultra-cytochemical reaction for lysyl-prolyl-dipeptidyl (amino) peptidase activity in a vacuolised yeast. L - lipoprotein M -mitochondria VA - vacuole W - cell wall microglobules are circled arrows indicate invaginations of plasmalemma filled with the reaction product of proteinase. Insert shows the reaction product in the periplasmic space underlying the yeast birth scar (open arrow). Arrows indicate the plasmalemma invaginations S -scar plug. The bar markers indicate 100 nm.

The extracellular enzymes of yeast are glycoproteins and include invertase and acid phosphatase. Strains of Saccharomyces carlsbergensis but not S, cerevisiae also possess melibiase. Yeast invertase has a minimum molecular weight of 27 000 contains 50 % mannan and 2-3 % glucosamine [43]. The precise location of the extracellular enzymes is still uncertain. They may occupy the space (periplasm) between the membrane and the bulk of the wall components, be anchored to the membrane via protein or covalently associated with other mannan or even glucan molecules. 

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