Sequence Determines Structure, Structure Determines Function

The previous lesson described the basic steps of gene expression. The transcription and translation processes are fundamental to the production of all proteins found in all living things. That is why discovering the sequence of a gene can allow geneticists to predict the amino acid sequence of the protein that gene encodes. Remember, a completely synthesized protein will be hundreds of amino acids long and thus an average gene will have two or three thousand nucleotides in it’s sequence. As more is learned about how amino acid sequence dictates protein function, the value of gene sequence information to biologists increases. In this lesson we will apply our understanding of the gene expression process to understanding the genetic basis of resistance to ALS herbicides in plants.

Fig. 1. A protein's function is dependent upon how it folds into a shape. The folding is a result of the sequence of amino acids present after translation. (Image by D. Lee)

The ALS enzyme is about 800 amino acids. The specific order of amino acids in the protein chain dictates how the molecule can be folded up into a secondary structure (Fig. 1). This is because each amino acid has a different chemistry that influences it’s interaction with other amino acids within the protein chain. The protein’s secondary structure determines the function of the protein. We can relate this principle to functional objects that are more familiar to us. Why would someone need a set of wrenches? Every tool in the set is made of the same materials. However, the specific shape and dimensions of each tool allow it to perform a specific function. The ALS enzyme functions to catalyze reactions for amino acid synthesis in the  chloroplast in a plant cell. Therefore the plant ALS enzyme structure signals the cell to transport the enzyme to the chloroplast and then allows the enzyme to catalyze the formation of a bond between amino acid precursor molecules. Clearly it was important to assemble the amino acids properly when building the ALS enzyme to allow it to function properly.