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Chapter 25 : Regulation of Urease for Acid Habitation

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Abstract:

Urease activity is essential for infection of animal models by . urease has a neutral pH optimum between 7.5 and 8.5 but essentially no activity below a pH of 4.5, and activity is lost irreversibly at this pH. The result of pH elevation due to outer urease activity would be that pH would rise to greater than 8.2 in the absence of acid or buffer, and then this neutralophile would not survive. The increase of activity is between 10- and 20-fold when external urease activity is present but can rise much more if surface urease is first washed off or selectively inactivated. Urease activity remains constant down to a pH between 2.5 and 3.0 and is detectable even at a pH of 2.0. This behavior corresponds to that of an acid resistance mechanism, wherein the neutralizing capacity is maintained over a broad range of acidic pH and is lost at pH levels where ammonia generation would generate a toxic environment for the organism. Since what is vital for the organism is the proton motive force (PMF) (pH/potential gradient) across the inner membrane, uncontrolled internal alkalinization would be unsafe, and the objective of creating a safe environment would be achieved by neutralization of the periplasm. The mechanism underlying this pH activation of urease is of vital interest in considering possible selective targets for monotherapy. Direct evidence for acid-activated urea transport was obtained by oocyte expression of one of the genes of the urease gene cluster, UreI.

Citation: Sachs G, Scott D, Weeks D, Rektorscheck M, Melchers K. 2001. Regulation of Urease for Acid Habitation, p 277-283. In Mobley H, Mendz G, Hazell S (ed), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch25

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Figures

Image of Figure 1
Figure 1

Relationship between membrane potential and external pH in The potential across the inner membrane of as a function of fixed medium pH using a fluorescent membrane potential dye, as detailed in reference 17. There is a reciprocal relationship between medium acidity and membrane potential as predicted from the chemiosmotic theory.

Citation: Sachs G, Scott D, Weeks D, Rektorscheck M, Melchers K. 2001. Regulation of Urease for Acid Habitation, p 277-283. In Mobley H, Mendz G, Hazell S (ed), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch25
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Image of Figure 2
Figure 2

pH profiles of urease activity in soluble urease (A) and intact bacteria (B) and profile of urea uptake in Urel-expressing oocytes (C). The activity of soluble or surface urease as a function of medium pH is shown. In contrast to the pH profile of free urease as shown in panel A, in intact bacteria (B) there is little activity at neutral pH, and a large increase at pH below 6.5 is interpreted as acid activation of intrabacterial urease. In panel C, urea uptake in Urel-expressing oocytes as a function of medium pH shows strong similarity to activation of urease in intact bacteria shown in panel B.

Citation: Sachs G, Scott D, Weeks D, Rektorscheck M, Melchers K. 2001. Regulation of Urease for Acid Habitation, p 277-283. In Mobley H, Mendz G, Hazell S (ed), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch25
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Image of Figure 3
Figure 3

pH profile of urease activity in UreI-negative mutants. The activity of urease in UreI-negative mutants as a function of pH (●) shows that the enzyme has lost its acid activation in the mutants. Full urease activity is restored by lysis or the addition of 0.01% CE, owing to the disruption of the bacterial inner membrane and this also shows the nonpolar nature of the mutant strain (○).

Citation: Sachs G, Scott D, Weeks D, Rektorscheck M, Melchers K. 2001. Regulation of Urease for Acid Habitation, p 277-283. In Mobley H, Mendz G, Hazell S (ed), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch25
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Image of Figure 4
Figure 4

Confocal image of A confocal image of incubated in the presence of BCECF-free acid at a pH of 5.5 immediately following the addition of urea. This dye does not penetrate the inner membrane and increases fluorescence with an increase of pH. The halo of increased fluorescence at the periphery of the organism defines the site of elevation of pH as being initially in the periplasm.

Citation: Sachs G, Scott D, Weeks D, Rektorscheck M, Melchers K. 2001. Regulation of Urease for Acid Habitation, p 277-283. In Mobley H, Mendz G, Hazell S (ed), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch25
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Image of Figure 5
Figure 5

A model illustrating the function of UreI. UreI is shown as a proton-gated urea channel providing urea access to intrabacterial urease under acidic conditions due to protonation of His-123. Two additional histidines and several carboxylic acids in the periplasmic loops may also participate in the gating response.

Citation: Sachs G, Scott D, Weeks D, Rektorscheck M, Melchers K. 2001. Regulation of Urease for Acid Habitation, p 277-283. In Mobley H, Mendz G, Hazell S (ed), . ASM Press, Washington, DC. doi: 10.1128/9781555818005.ch25
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References

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