Agriculture Reference
In-Depth Information
common element in the crust, after oxygen, silicon and aluminium. As a transition
metal, Fe can easily accept and donate electrons. Thus its oxidation state can vary in
a broad range between
2 and +6, but the most common under current atmospheric
conditions are +2 (ferrous iron) and +3 (ferric iron). This redox property of Fe and
its capability to form complexes with different ligands make this element indis-
pensable for different biological processes in all the living organisms. Indeed
several proteins (Fe-proteins) found in different organisms rely on Fe as a cofactor
for proper functioning. Fe is also essential for plant metabolism, where it partici-
pates in vital cellular functions such as photosynthesis, respiration and chlorophyll
biosynthesis.
Despite its essential role for life, an excess of free Fe can be detrimental to the
cell because it can react with oxygen catalysing the formation of reactive oxygen
species (ROS) such as superoxide (O
2
∙
) and hydroxyl radical (OH
∙
) via the Fenton
reaction (reviewed by Hell and Stephan
2003
):
Fe
3
þ
þ
O
2
!
Fe
2
þ
þ
O
2
Fe
2
þ
þ
Fe
3
þ
þ
OH
þ
OH
H
2
O
2
!
O
2
þ
OH
þ
OH
Resulting in
H
2
O
2
!
O
2
þ
:
These radicals constitute a severe danger for the cell particularly the hydroxyl
radical, which is very reactive and can indiscriminately oxidise DNA, polyunsatu-
rated fatty acids in lipids (lipid peroxidation), amino acids in proteins and sugars.
To avoid potentially toxic reactions, protein-bound Fe is found incorporated into
structures such as heme or coordinated with sulfur (S) to form Fe-S cluster. Heme
contains a Fe atom in the centre of a large heterocyclic organic ring, the porphyrin,
made of four pyrrolic groups joined by methine bridges. Among heme proteins, a
fundamental role is played by hemoglobin and myoglobin in vertebrates as they
contain a Fe atom which binds to oxygen. The most common heme proteins in
plants are cytochromes, which participate in the electron transport process of
mitochondria and chloroplasts. Other heme proteins are catalase and peroxidases
that are involved in scavenging of ROS.
Fe-S clusters are versatile and ubiquitous cofactors of many different enzymes
that participate in respiration, photosynthesis, DNA repair and replication, sulfur
and nitrogen assimilation and ribosome biosynthesis (Balk and Pilon
2011
). They
are formed from Fe atoms and sulfur in the form of acid-labile sulfide and are bound
to proteins via the sulfhydryl groups of cysteine residues. Different forms are found
in plants, the most common are the 2Fe-2S and 4Fe-4S bound to four cysteine (Cys)
residues. Other types include the 2Fe-2S Rieske-type cluster coordinated by 2 Cys
and 2 His residues and 3Fe-4S liganded by 3 Cys (reviewed by Couturier
et al.
2013
).
Toxicity of free Fe ions may be avoided by chelation by different compounds
such as the non-proteinogenic amino acid nicotianamine, citrate and the storage via
ferritin proteins.
