Transport, Storage, and Homeostasis of Metal Ions - Biological Inorganic Chemistry - page 160

FIGURE 8.5 Copper homeostasis in Enterococcus hirae. Copper homeostasis in Lactococcus lactis. How copper enters the cell is unknown.
Excess cytoplasmic copper binds to CopZ, which can then donate Cu þ to either the copper ATPases for export or the CopR repressor to induce
transcription. In low-copper conditions, a CopR dimer in the zinc form is bound to the cop box in front of the copRZA operon and the copB gene.
When CopZ donates Cu þ to CopR, one Zn 2 þ per CopR monomer is replaced by two Cu þ , with concomitant release of CopR from the
promoters and induction of transcription of the downstream genes. CopA then accomplishes copper export from the cytoplasm. The function of
CopB is unknown. (From Solioz, Abicht, Mermod, & Mancini, 2010 . Copyright 2010 with permission from Springer Verlag.)

Bacterial copper proteins are found only in the plasma membrane (Gram-positive bacteria) or in the plasma
membrane and the periplasm (Gram-negative bacteria), not in the bacterial cytoplasm. However, cyanobacteria do
have copper proteins in their cytoplasm. These important photosynthetic bacteria require copper for plastocyanin,
which plays a critical role in the photosynthetic electron transport chain. Both plastocyanin and cytochrome c
oxidase are found in the thylakoid compartments within the cytoplasm. In Synechocystis, the Cu(I) P 1B -ATPase
CtaA imports Cu(I). A second ATPase, PacS, imports Cu(I) into the thylakoid, and the Atx1-like copper chaperone
ScAtx1 is believed to deliver Cu(I) from CtaA to PacS ( Fig. 8.6 ) .

FIGURE 8.6 Copper trafficking pathways in the cyanobacterium Synechocystis.

Inmany bacterial species, zinc storage is apparently not amajormechanismin attaining homeostasis, the exception
being cyanobacteria, which detoxify and store zinc in a metallothionein. A more common way of ridding the cell of
excess zinc is by exporting it. The importance of this is clearly illustrated by the highly Zn 2 þ -resistant bacterium,
Ralstonia metallidurans, isolated from a decantation tank in a zinc factory, and which has a minimal inhibitory Zn 2 þ