Chemistry Reference
In-Depth Information
Chapter 15
Nickel and Cobalt: Evolutionary Relics
Introduction
297
Nickel Enzymes
297
Methyl-coenzyme M Reductase
302
Cobalamine and Cobalt Proteins
303
B
12
-dependent Isomerases
303
B
12
-dependent Methyltransferases
306
Noncorrin Co-containing Enzymes
308
INTRODUCTION
When one examines the kinds of reactions catalysed by nickel and cobalt enzymes and their evolutionary
distribution, one arrives at the conclusion that these two elements were particularly important in the metabolism of
chemicals particularly abundant in the pre-oxygen evolutionary era, like methane, carbon monoxide, and
hydrogen. This is reflected in the high levels of both elements in a number of anaerobic bacteria. In contrast, the
level of both metals in mammalian serum is less than 100-fold that of zinc, iron, or copper. Nonetheless, cobalt,
through its involvement in a number of important vitamin B
12
-dependent enzymes continued to be used in higher
organisms, including mammals. In contrast, with the exception of the plant enzyme urease, nickel proteins are
virtually unknown in higher eukaryotes.
Both nickel and cobalt, together with iron, have the characteristic that they are electron rich. Furthermore, in
lower oxidation states some of their 3d electrons are forced into exposed
-) orbitals: the outcome is that
tetragonal Co(II) or Ni(III) are reactive free radicals, able to give or take a single electron, in the same way as
s
s
-(or
p
-organic free radicals. So, cobalt functions in free-radical reactions, such as the transformation of ribonucleotides
into their corresponding deoxy derivatives, just like iron. The participation of cobalt or nickel in acid
base
chemistry could easily be replaced by zinc, while any redox functions in a post-oxygen world could readily be
substituted by iron, copper, or manganese, all of which were much more bioavailable. So nickel, in particular, but
also cobalt became the lost leaders of the post-photosynthetic supermarket shelf of bio-metals.
We begin by considering nickel enzymes, and then move on to cobalt, concentrating on enzymes with cobalamine
cofactors, including also some noncorrin cobalt enzymes. For reviews, see Bannerjee and Ragsdale, 2003; Brown,
2005; Hegg, 2004; Kobayashi and Shimizu, 1999;Mulrooney andHausinger, 2003; Ragsdale, 1998, 2004, 2006, 2009.
e
NICKEL ENZYMES
Seven of the eight known Ni enzymes (
Table 15.1
)
are involved in the use and/or production of gases (CO, CO
2
,
CH
4
,H
2
,NH
3
, and O
2
) which all play important roles in the global cycles of carbon, nitrogen, and oxygen
(
Ragsdale, 2007, 2009
) (Chapter 18). Urease, the first Ni enzyme to be discovered, produces NH
3
, acireductone
dioxygenase (ARD) produces CO, SOD generates O
2
from superoxide, hydrogenase rather generates or utilises
