Organic compatible solutes of halotolerant and halophilic microorganisms

Abstract

Microorganisms that adapt to moderate and high salt environments use a variety of solutes, organic and inorganic, to counter external osmotic pressure. The organic solutes can be zwitterionic, noncharged, or anionic (along with an inorganic cation such as K(+)). The range of solutes, their diverse biosynthetic pathways, and physical properties of the solutes that effect molecular stability are reviewed.

Figure 1

Zwitterionic organic osmolytes detected in bacteria and archaea.

Figure 2

Uncharged organic osmolytes detected in bacteria and archaea.

Figure 3

Anionic organic osmolytes containing carboxylates that have been detected in bacteria and archaea.

Figure 4

Anionic organic osmolytes containing phosphate or sulfate moieties that have been detected in bacteria and archaea.

Figure 5

Pathways for synthesizing betaine in bacteria and archaea.

Figure 6

Biosynthetic pathway for ectoine.

Figure 7

Proposed biosynthetic pathway for β-glutamate.

Figure 8

Proposed pathway for DIP biosynthesis in hyperthermophilic organisms.

Figure 9

Two pathways exist for α-mannosylglycerate biosynthesis. In (I) GDP-mannose is directly converted to mannosylglycerate. In (II), the GDP-mannose condenses with 3-phosphoglycerate to for mannosyl-3-phosphoglycerate, which is subsequently dephosphorylated to form mannosylglycerate.

Figure 10

Proposed biosynthesis of cDPG as a pathway linked to gluconeogenesis through 2-PG and 3-PG. The dashed lines indicate the reversible cDPGS reaction of Methanothermus fervidus. The solid lines show cDPG and 2,3-DPG interconversions and illustrate the irreversible nature of the cDPGS in Methanobacter thermoautotrophicus.

Figure 11

(A) Exclusion of solutes from the surface of a protein increases the concentration of solute in the bulk solution, which in turn increases water surface tension generating osmotic pressure that drives a protein to retain a more compact structure. (B) Osmolytes stabilize proteins to thermal denaturation by differentially raising the energy level of the unfolded state: U, unfolded state; F, folded state; aq, aqueous solution; S, solution containing osmolytes.