Chemistry Reference
In-Depth Information
gradients to drive ATP synthesis. While we can transfer electrons using organic molecules like flavins, redox metal
ions like iron and copper are much better adapted to this. We need to find ways of amplifying signals, arriving at
the cell membrane at nanomolar concentrations, but which result in millimolar intracellular responses. As we
move from unicellular organisms to more complex multicellular organisms, we need to generate transmembrane
electrical potentials so that we can transmit messages in the form of electrical signals, sometimes over quite long
distances. For almost all of these purposes, large, cumbersome and bulky proteins are clearly not the answer. But,
perhaps above all else, we must enable the proteins which we call enzymes to catalyse reactions, many of which
would quite simply be impossible if we relied solely on organic molecules.
So, if these six elements alone do not enable life as we know it to exist, in its multiple and varied forms
what
other elements do we require? Traditionally, whereas organic chemistry concerns compounds of biological origin,
inorganic chemistry concerns the properties and behaviour of inorganic compounds, considered to be of mineral
origin
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inorganic chemistry in French was previously called 'chimie mine´rale'(mineral chemistry
2
)
. In more
recent times, the boundaries between inorganic and organic have become more blurred
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many inorganic
compounds contain organic ligands, while, as mentioned earlier, some carbon-containing compounds are tradi-
tionally considered inorganic, and many organic compounds contain metals. As we will see in the next section, in
the course of evolution, Nature has selected constituents not only from the organic world, but also from the
inorganic world to construct living organisms. Many of these are metals, elements to the left of the periodic table,
which readily lose their valence electrons to form cations.
There is an interesting historical illustration of this requirement for metals in catalysis. The celebrated German
chemist Richard Willst¨tter (Chemistry Nobel Prize, 1915) proposed that enzymes were not proteins
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in his view,
the protein was only a carrier for the veritable catalytic centre (he called the protein “nur ein tra¨ger Substanz”). In
1929, James Sumner accidentally left a preparation of urease from jack bean (the enzyme which catalyses the
decomposition of urea to ammonia and carbon dioxide) on a laboratory table overnight. The night was cold, and to
his surprise, the following morning, he found that the protein had crystallised. Together with John Northrop, who
crystallised pepsin and trypsin, the conclusive proof of the protein nature of enzymes was thereby established
(they both received the Chemistry Nobel Prize in 1946). Although their discovery appeared to have disproved
Willst
¨
tter's theory, he was vindicated some 50 years later by the demonstration that urease is in fact a nickel-
dependent enzyme, and that when the Ni is removed, urease loses its catalytic activity. Of course, with the benefit
of hindsight, we can see that both viewpoints were correct. The protein is indeed a carrier for the Ni, but a carrier
which provides the right coordination sphere
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to bind the Ni in the right conformation, as well as creating the right
environment for the molecular recognition of the substrates, urea and water, and their binding in the right
orientation to enable the di-metallic nickel site to carry out its catalysis (see Chapter 15 for more details).
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WHAT ARE THE ESSENTIAL ELEMENTS AND THE ESSENTIAL METAL IONS?
Just six elements
make up almost 98.5% of the
elemental composition of the human body by weight. Just 11 elements account for 99.9% of the human body (the
additional five are potassium, sulfur, sodium, magnesium, and chlorine). However, as we will see shortly, we can
identify between 22 and 30 elements which are required by some, if not all, living organisms. Many of these are
metals: some of them, like sodium, potassium, calcium, and magnesium, are present in quite large concentrations,
and are known as 'bulk elements'. Indeed, these four cations constitute nearly 99% of the metal ion content of the
human body. Others, like cobalt, copper, iron, manganese, molybdenum, and zinc, are known as 'trace elements',
with dietary requirements that are much lower than the bulk elements; yet, they are no less indispensable for
human life.
oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus
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2. As illustrated by the avant-garde translation of the title of Steve Lippard and Jeremy Berg's book 'Principles of Bioinorganic Chemistry'
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'Biochimie Mine´rale'.
3. See the Glossary for explanations concerning specialised terms like this.
