Proteins are polypeptides folded into specific, complex three-dimensional shapes. Generally, hydrophobic amino acids pack into the interior of the protein while hydrophilic amino acid side chains end up on the surface where they can interact with water. The amino acid side chains pack together tightly in the interior: there are no spaces. It is the three-dimensional structure of a particular protein that allows it to carry out its role in the cell. The shape can be fully defined by stating the position and orientation of each amino acid, and such knowledge lets us produce representations of the protein (Fig. 9.12). However, when discussing protein structure, it is helpful to think in terms of a series of different levels of complexity.
The primary structure of a protein is the sequence of amino acids. Lysozyme is an enzyme that attacks bacterial cell walls. It is found in secretions such as tears and in the white of eggs. Lysozyme has the following primary structure:
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