Chemistry Reference
In-Depth Information
P-cluster of nitrogenases, the three types of metal clusters found in microbial hydrogenases, the unusual common
structural features of which include CO ligands, and the Cu
Z
cluster of microbial nitrous oxidases. The biosyn-
thesis of some of these cofactors is discussed later in this chapter.
The unique case of microbial siderophores, which are low-molecular-weight iron chelators, will be discussed
right at the end of this chapter.
INSERTION OF METAL IONS INTO METALLOPROTEINS
At this point, we might ask what might seem like an obvious question. Are the metal sites, which we have dis-
cussed above pre-formed (i.e., does the binding site already exist when the protein is synthesised), or does metal
binding result in a major change in the conformation of the protein? In FbpA and the transferrins which we
encountered earlier, there is clear evidence for a conformational change between an 'open' (metal-free) and
a 'closed' (metal-bound) form of the protein. Fe binding and release is proposed to function via a 'Venus fly trap'
mechanism where large-scale movements of the two domains result in the opening or closing of the iron-binding
site which lies between them, as we discuss in greater detail in Chapter 8.
As we will also see in Chapter 8, free copper levels are extremely low within cells because the copper is bound
to a family of metallochaperones
5
which are subsequently involved in the incorporation of copper into copper-
containing proteins. The mechanism proposed for copper insertion into the Cu/Zn superoxide dismutase, SOD1, is
presented in
Fig. 4.7
, and appears to use an already-preformed Cu-binding site. The copper chaperone CCS
acquires copper as Cu
รพ
from a copper transporter and then docks with the reduced dithiol form of SOD1 (steps I
FIGURE 4.7
Proposed mechanism of copper insertion into SOD1 by its metallochaperone, CCS. The copper chaperone acquires copper
through unknown routes and then docks with a disulfide reduced form of SOD1 (steps I and II). This complex is inert to further reaction unless
exposed to oxygen or superoxide (step III), at which point a disulfide-linked heterodimeric intermediate forms. This complex undergoes
disulfide isomerisation to an intramolecular disulfide in SOD1 (step IV). Copper is transferred at some point after introduction of oxygen and
the mature monomer is proposed to be released from CCS.
(From
Culotta, Yang, & O'Halloran, 2006
. Copyright 2006, with permission from
Elsevier.)
5. Chaperones were persons who, for the sake of propriety, accompanied young unmarried ladies in public, as guide and protector.
