The Biology Project: Biochemistry

Large Molecules Problem Set

Problem 3: Tertiary structure of a protein

Tutorial to help answer the question

The tertiary structure of a protein refers to the:

A. Sequence of amino acids

B. Presence of alpha-helices or beta-sheets

C. Unique three dimensional folding of the molecule

D. Interactions of a protein with other subunits of enzymes

E. Interaction of a protein with a nucleic acid


It is convenient to describe protein structure in terms of 4 different aspects of covalent structure and folding patterns. The different levels of protein structure are known as primary, secondary, tertiary, and quaternary structure.
Primary Structure of Proteins
The primary structure is the sequence of amino acids that make up a polypeptide chain. 20 different amino acids are found in proteins. The exact order of the amino acids in a specific protein is the primary sequence for that protein. Illustration of a polypeptide strand as described

Secondary Structure of Proteins
Protein secondary structure refers to regular, repeated patterns of folding of the protein backbone. The two most common folding patterns are the alpha helix and the beta sheet.
Alpha Helix
In an alpha helix, the polypeptide backbone coils around an imaginary helix axis in clockwise direction.

In this illustration, only the N-C-CO backbone atoms are shown. Note the coiling of the backbone around an imaginary axis down the center of the helix.

Illustration of a alph helix as described
Beta sheet
In the beta sheet secondary structure, the polypeptide backbone is nearly fully extended. The R-groups (not shown) are alternately pointed above and then below the extended backbone.
Illustration of a beta sheet as described

Tertiary Structure of Proteins
Tertiary structure refers to the overall folding of the entire polypeptide chain into a specific 3D shape. The tertiary structure of enzymes is often a compact, globular shape. Illustration of a tertiary structure of proteins as described
Tertiary structure of the triose phosphate isomerase (TPI) molecule.
Space-filling view of the backbone of TPI molecule linking to a rotating quickTime movie of same

Ribbon view of the backbone of TPI molecule and a link to a QuickTime movie of the same rotating

Get QuickTime botton

Quaternary Structure of Proteins
Many proteins are formed from more than one polypeptide chain. The quaternary structure describes the way in which the different subunits are packed together to form the overall structure of the protein. For example, the human hemoglobin molecule shown below is made of four subunits.
Space-filling view of human hemoglobin molecule Ribbon view of a human hemoglobin molecule

problem 3 answer problem 4

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