Protein Structure

Types of Representations

Proteins may be represented in several ways. Below you can choose from two views of insulin.

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Primary Structure

The primary structure of a protein is its linear sequence of specific amino acids. A single amino acid is shown to the right. The "side chain" is the portion that is different in each of the 20 different amino acids.

Peptide bonds hold the adjacent amino acids together in the polypeptide chain. The figure to the right shows two amino acids held together by a peptide bond (in red). Note the polar nature of the region around the peptide bond.

Secondary Structures

A local region of the polypeptide chain may fold into either an alpha-helix or a beta sheet. These special structures comprise the secondary structure.

Alpha-helix Proteins

The secondary structure of some proteins is essentially all alpha-helices.

Myoglobin is good example of this type. This second view of myoglobin shows the hydrogen bonds that hold the alpha helices together.

QUESTION: Do you know why hydrogen bonds tend to form so frequently in a polypeptide chain?

Fill in your answer below:

Beta-sheet Proteins

Some proteins are largely formed from beta-sheets. Antibodies and T cell receptors fall in this category. Shown here are the constant and variable domains of one polypeptide chain of a T cell receptor. (The membrane and intracellular portions are not shown.)

Alpha-helix and Beta-sheet Proteins

Many proteins have both alpha-helices and beta-sheets. The large domain of hexokinase, which phosphorylates glucose, is an alpha/beta structure. Glucose is shown at the center of the enzyme.

Tertiary Structure

The tertiary structure of a protein refers to the actual three dimensional structure of the polypeptide chain. A number of forces act to hold the polypeptide chain in this final configuration:

Quaternary Structures

Some proteins have a quaternary structure; that is, they are comprised of two or more polypeptide chains. Each polypeptide chain in such a protein is called a subunit. Hemoglobin is one example. Each of the four polypeptide chains is similar to myoglobin. Note the presence of the oxygen binding heme groups, which are discussed below.

Collagen is a widespread connective tissue protein, which consists of three polypeptide chains. Only a short segment in the middle of this large, fibrous extracellular molecule is shown.

Prosthetic Groups

Myoglobin is a protein with a prosthetic group called a heme. It contains an iron ion that binds oxygen. You may look at the space-filling representation or further type of representation in which bonds are shown as sticks

Quick Quiz

Fill in Answer Correct False Correct Answer
Name a type of bond that joins adjacent amino acids in a protein.
Name the type of bond that primarily is responsible for holding a polypeptide chain in an alpha helix or beta sheet.
What level of structure refers to the exact three dimensional shape of a single polypeptide chain?
Which amino acid in this diagram has the most nonpolar side chain?

(Spelling must be correct)
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