Figure 2.17 - Four Levels of Protein StructureĪt this point, we should provide a couple of definitions. We shall examine protein structure at four distinct levels (Figure 2.17) - 1) how sequence of the amino acids in a protein (primary structure) gives identity and characteristics to a protein (Figure 2.18) 2) how local interactions between one part of the polypeptide backbone and another affect protein shape (secondary structure) 3) how the polypeptide chain of a protein can fold to allow amino acids to interact with each other that are not close in primary structure (tertiary structure) and 4) how different polypeptide chains interact with each other within a multi-subunit protein (quaternary structure). This conformation, in turn, will determine the function of the protein. The significance of the unique sequence, or order, of amino acids, known as the protein’s primary structure, is that it dictates the 3-D conformation the folded protein will have. Most proteins are much larger than this, making the possible number of proteins with unique amino acid sequences unimaginably huge. If we do the calculation for a short peptide of 10 amino acids, we arrive at an enormous 10,240,000,000,000 combinations.
It becomes apparent that even a dipeptide made of just two amino acids joined together gives us 20 2 = 400 different combinations. To get a sense for the diversity of proteins that can be made using 20 different amino acids, consider that the number of different combinations possible with 20 amino acids is 20 n, where n=the number of amino acids in the chain. Proteins are made by linking together amino acids, with each protein having a characteristic and unique amino acid sequence. The versatility of proteins arises because of their varied structures. Life, as we know it, would not exist if there were no proteins. Among other things, protein enzymes catalyze the vast majority of cellular reactions, mediate signaling, give structure both to cells and to multicellular organisms, and exert control over the expression of genes. Virtually everything that goes on inside of cells happens as a result of the actions of proteins.
AMINO ACID SEQUENCE FOR FREE
The entire textbook is available for free from the authors at
0 kommentar(er)
