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PROTEINS AND NUCLEIC ACIDS
Amino acids and Proteins
Plants contain a lot less nitrogen than animals, whose bodies are mostly made up of protein.
Although plants have little in the way of structural protein, nitrogen compounds are essential for
their life. In fact animals are as dependent on plants for their organic nitrogen as for their
carbohydrate foods. Amino acids are the main low molecular weight organic nitrogen
compounds. These are variations on a basic pattern:
Amino acids are the building blocks for proteins in
which peptide bonds (a special kind of amide bond)
join the amino and carboxyl groups of adjacent
monomers: The sequence of amino acids defines the
primary structure of a protein. The amino acid chain is
typically coiled in an alpha-helix which defines the
secondary structure. The helix is then folded in a
specific pattern which is the tertiary structure.
Enzymes
Most of the proteins in plants are enzymes, catalysts
that carry out all of the chemical changes involved in
plant growth. There is at least one enzyme specifically
responsible for every step in respiration,
photosynthesis, gene replication, information
processing and building cell structure. The specific
reaction is determined by the primary amino acid
sequence of the protein, but it is the secondary and
tertiary structure that is important for catalytic function.
The way that the protein is folded creates an active
site with an affinity for a specific substrate molecule.
When the molecule binds to the active site it is
stretched or twisted in a way that encourages it to go
through the reaction. Thus invertase stretches the
glycosidic bonds in sucrose, allowing hydrogen and
hydroxyl from water to get in and split glucose and
sucrose away.
http://www.hcs.ohio-state.edu/hcs300/biochem2.htm
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Nucleic acids and
nucleotides
Nucleotides are another
group of nitrogen containing
compounds with special
functions in the cell. They are
mostly based on five organic
bases. Two of these, thymine
and uracil (U) differ only in
the presence or absence of
one methyl group:
These are often combined
with
ribose (a pentose sugar) and one or more phosphate groups. Chemical energy in all cells is
commonly handled through ATP. The energy is associated with the chain of three phosphates
and is released by stripping them off, usually one at a time.
Chemical reducing power in the cell is associated with NAD which carries a positive charge in
the oxidized state and is written as NADH when reduced. Sometimes a related compound NADP
is used in place of NAD. Polymers based on similar nucleotides are used in information storage
and processing. As we see with the viruses it is impossible to have anything resembling life
without this function. Information is usually stored in DNA in which the ribose sugar is modified
(deoxyribose) and two chains of nucleotides are lined up side by side (but running in opposite
directions). The backbone of each chain is an alternating sequence of sugar and phosphate. The
bases are attached to the sugars and the bases line up so that every time there is a G on one
chain there is a C on the other; when A is on one T is on the other.
http://www.hcs.ohio-state.edu/hcs300/biochem2.htm
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This arrangement gives the cell two
complementary copies of its genetic material.
When it is time to reproduce the strands are
separated and the complementary nucleotides
are brought in for each strand. Strands are
always copied in one direction, so one strand
can be copied continuously but the other must
be copied in fragments that are joined up later.
Normally DNA exi sts as (the famous) double
helix. This has to be unwound for replication
DNA also has to be unwound and the strands
separated in order to read off information as
RNA. This is copied from one strand of the DNA
and uses the same base pairing as in DNA,
except that RNA contains uracil (U) in place of
thymine (T). Proteins are made by transcribing
messenger RNA(mRNA) from DNA and
translating it into aan amino acid sequence on a
ribosome. Ribosomes are also made of RNA
(rRNA) and the process involves transfer RNA
(tRNA) to make the temporary link between
amino acids and mRNA.
http://www.hcs.ohio-state.edu/hcs300/biochem2.htm
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http://www.hcs.ohio-state.edu/hcs300/biochem2.htm
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