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Inhibitors of Aromatic Amino Acid Biosynthesis

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Importance of Proteins

There are many stages of growth and development which take place throughout a plant’s life. If the particular plant is a crop, then producers want these stages to occur efficiently to obtain high yields. If the plant is a weed, reducing the crop’s yield potential, then producers look for ways to inhibit these cellular processes.

Herbicide proteins are the critical factors in both of these scenerios. They are important macro-molecules that participate in every aspect of plant growth and development. Proteins are involved in processes such as catalyzing chemical reactions (enzymes), facilitating membrane transport, intracellular structure and energy generating reactions involving electron transport, just to name a few. Compared to animals, plants contain low levels of protein due to the large amount of structural carbohydrate (cellulose) that compose most of a plant’s structure.

To better understand plant growth and development we need to take a closer look at these protein molecules. Proteins are constructed from even smaller molecules called amino acids. In Figure 1, the general molecular structure of amino acids is illustrated. Notice that all amino acids contain both an amino group and a carboxyl group; however, individual amino acids differ at the 'R' group.



Figure 1: All amino acids contain 3 parts, an amino group NH3+),a carboxyl group (COO-)and an 'R' group. The 'R' group varies among amino acids. Figure 2: Shows the structure for a specific amino acid, tryptophan.
Figure 3:Three structures of proteins are illustrated (A) primary structure, its amino acid sequence; (B) secondary structure, polypeptide coiling or folding; (C) tertiary structure, the overall shape of the polypeptide.
 

Unlike animals, plants do not derive amino acids by consuming other organisms. Therefore, all 20 of these amino acids must be synthesized by the plant. A challenge lies in the fact that proteins have a finite life span and must be constantly translated from m-RNA in order for plant growth and development to continue. This means that there must be a ready supply of all 20 amino acids for protein synthesis and ultimately plant growth and development occur.

A protein is created when a series of amino acids are bound together. The arrangement of the amino acids can be described at 3 different levels: primary structure, secondary structure and tertiary structure. (Fig 3) The primary structure is simply the order in which the amino acids are bound together. This specific order is initially encoded in the DNA sequence of the gene that provides the blueprint for the protein (Fig 3-A).

The secondary structure of a protein describes the way the string of amino acids fold (Fig 3-B). The specific bonding interactions among amino acids will determine if it is a coil or folding arrangement for these interactions to take place based on the primary structure.

Finally, the tertiary structure of a protein describes the overall shape of the protein. It is a protein’s shape which ultimately determines a protein’s function (Fig 3-C).

Therefore, changing a gene’s sequence can change the primary structure of a protein, the amino acid sequence. This in turn may change a protein’s secondary structure and then it’s shape, or tertiary structure. With an altered shape, the protein may function differently or may not function at all.

For example, plant’s resistant to glyphosate have EPSP synthase enzymes with sightly different amino acid sequence than susceptible plants. This is due to a simple DNA mutation. The details will be explained later.

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