Question

If a protein common to two different species has many amino acid differences, what can be concluded?

Answer 1

From a chemical point of view, proteins are by far the most structurally complex and functionally sophisticated molecules known. This is perhaps not surprising, once one realizes that the structure and chemistry of each protein has been developed and fine-tuned over billions of years of evolutionary history. We start this chapter by considering how the location of each amino acid in the long string of amino acids that forms a protein determines its three-dimensional shape. We will then use this understanding of protein structure at the atomic level to describe how the precise shape of each protein molecule determines its function in a cell.

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The Shape of a Protein Is Specified by Its Amino Acid Sequence

Recall from Chapter 2 that there are 20 types of amino acids in proteins, each with different chemical properties. A protein molecule is made from a long chain of these amino acids, each linked to its neighbor through a covalent peptide bond (Figure 3-1). Proteins are therefore also known as polypeptides. Each type of protein has a unique sequence of amino acids, exactly the same from one molecule to the next. Many thousands of different proteins are known, each with its own particular amino acid

A peptide bond. This covalent bond forms when the carbon atom from the carboxyl group of one amino acid shares electrons with the nitrogen atom (blue) from the amino group of a second amino acid. As indicated, a molecule of water is lost in this condensation

The repeating sequence of atoms along the core of the polypeptide chain is referred to as the polypeptide backbone. Attached to this repetitive chain are those portions of the amino acids that are not involved in making a peptide bond and which give each amino acid its unique properties: the 20 different amino acid side chains .Some of these side chains are nonpolar and hydrophobic (“water-fearing”), others are negatively or positively charged, some are reactive, and so on.