Alberta Ingenuity & CMASTE
Case Study:
The Oxidation of DSP4 in Chloroplasts
What is an Amino Acid?
An amino acid is an organic compound which contains both an amine functional
group and a carboxyl functional group. There are 20 different amino acids in
the human body. The simplest amino acid is glycine (NH2CH2COOH). All amino
acids have the same base structure as glycine but a hydrogen is replaced by a
combination of hydrogen, carbon, nitrogen, oxygen and sulfur. Cysteine
(HOOCCH(NH2)CH2SH) and methionine (HOOCCH(NH2)CH2CH2SCH3) are the only
two amino acids which contain sulfur.
Forming Proteins
Amino acids are the building blocks for protein. Through the process of
condensation polymerization the carboxyl group of one amino acid reacts with
the amine end of another to form water and a dipeptide. Each pair of amino
acids are linked together by a peptide bond and giant chains are formed.
Figure C Formation of a
peptide bond
between 2
amino acids.
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Roles of Proteins
When thousands of the amino acids
are joined together in a chain, a
protein is produced. Proteins are
essential to organisms and take part
in many cell functions. Sometimes
proteins are critical to the structure
of the cell, such as the proteins in
the cytoskeleton. Enzymes are
proteins which catalyze reactions.
There are also proteins which serve
in cell signaling and immune
response.
Figure D - The enzyme DSP4(this picture
is not DSP4, but insert a picture like this)
DSP4
At AICSS, scientists are looking at an enzyme, a phosphatase (DSP4), in plants.
Phosphatases add phosphate groups to proteins serving to activate or
deactivate a protein or allow a protein- protein interaction. DSP4 is located in
the chloroplasts along with starch. DSP4 has been shown to control the
accumulation of starch in some plants. This project examines the properties of
the enzyme in the oxidized and reduced states. If DSP4 control the
accumulation of starch, it may assist in the management of the symptoms of
the growing population of Type II Diabetes sufferers.
Oxidation of Cysteine
One of the amino acids in DSP4 is cysteine. Cysteine is a strong reducing agent.
It can prevent the oxidation of other substances. Cysteine can react with itself
and oxygen to form an oxidized dimer by formation of a disulfide bond. Many
organic compounds are too complex to use oxidation numbers in deciding
whether oxidation or reduction has occurred. When looking at cysteine, one
must look at the changes in bonds to hydrogen and/or oxygen. Loss of bonds to
hydrogen or gain of bonds to an oxygen atom is oxidation. The environment
within a cell is too strongly reducing for disulfides to form, but in the extracellular environment, disulfides can form and play a key role in stabilizing
many such proteins.
Figure E. Formation of a disulfide bond through the oxidation of cysteine.
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Student Exercise:
1. Amino acids are formed with only six different atoms. Identify the atoms
present in amino acids.
2. Not every amino acid contains all six atoms. Sulfur is contained in only two
amino acids. List the two amino acids containing that atom.
3. Draw the two functional groups which are always present on an amino acid
and name them?
4. Proteins have many roles in the cell. Describe the three functions of
proteins?
5. Explain the role of phosphatase in a cell.
6. Draw the reaction between glycine and cysteine to form a dipeptide. Given
the products that are formed, what would you classify the reaction as?
7. In the cysteine molecule, it is impossible to determine oxidation numbers.
What evidence is there to support the fact that cysteine is oxidized to form
cystine?
8. The other amino acid containing sulfur is methionione. It is an essential
amino acid, meaning it cannot be produced in the body. Its structure is:
When compared to cysteine, what functional
groups are the same? What is the difference
between the two molecules?
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