Protein Structure and Function Flashcards

The shape of a protein can be described by four levels of structure: primary, secondary, tertiary and quaternary.

Primary structure is the unique and linear sequence of amino acids in a protein. It is the sequence in which amino acids are added to a growing polypeptide during translation.
With 20 different amino acids, the number of primary sequences is almost infinite.
It is the primary structure that determines how (and where) the polypeptide will fold to give a protein its shape. Thus, primary structure determines the higher levels of protein structure.
Small changes in primary structure can result in large changes in protein shape and function.

Secondary structure describes regions where the polypeptide is folded into localized shapes. There are two types of secondary structure (alpha helix and Beta pleated sheet).
The alpha helix is a delicate coil formed by hydrogen bonding between a hydrogen atom on one amino acid and an oxygen atom on the fourth amino acid away.
The beta sheet results from hydrogen bonding between different polypeptide chains or between different sections of the same polypeptide.

Tertiary structure is the overall shape of the protein. Most proteins (e.g. lysozyme, hemoglobin and insulin) have a compact, globular tertiary structure.
Some proteins are fibrous. Fibrous proteins like collagen (tendons, cartilage) and keratin (hair, feathers, horns, hoofs, etc.) have the alpha helix formation over their entire length. Other fibrous proteins like fibroin (the structural protein of silk) are dominated by beta sheets.
Tertiary structure is influenced by ionic bonds between opposite charged R-groups, hydrogen bonds between R-groups bearing opposite partial charges, and hydrophobic interactions resulting from the tendency of nonpolar R-groups to stay close together in an aqueous solution.

Quaternary structure occurs in proteins that are made up of more than one polypeptide chain.
Combining different polypeptides leads to a greater range of biological activity. Collagen, for example, is made of three subunits intertwined into a triple helix, and hemoglobin is made of four heme groups, each a different polypeptide.
An influence on the quaternary structure of some proteins is the presence of a prosthetic group: a small molecule that is not a peptide but that tightly binds to the protein and plays a crucial role in its function. For example, the four heme groups on a hemoglobin protein are prosthetic and they function to carry oxygen.
Proteins with prosthetic groups are called conjugated proteins.

Most proteins (e.g. lysozyme, hemoglobin and insulin) have a compact, globular tertiary structure. Enzymes are usually globular.
Some proteins are fibrous. Fibrous proteins like collagen (tendons, cartilage) and keratin (hair, feathers, horns, hoofs, etc.) have the alpha helix formation over their entire length. Other fibrous proteins like fibroin (the structural protein of silk) are dominated by beta sheets.

The 20 different amino acids vary in their R groups; some R groups are non-polar, others are polar.
Polar amino acids have R groups that carry either a (+) or a (-) charge.
Polar amino acids are hydrophilic and non-polar amino acids are hydrophobic.
Hydrophobic R-groups stay close together in water.
Proteins with a lot of polar amino acids are soluble in water, and those with many non-polar amino acids do not dissolve in water.
The hydrophilic and hydrophobic properties of amino acids cause proteins to twist into useful shapes. This ability of proteins is important for cellular membranes.
Membrane proteins are firmly anchored in the phospholipid bilayer because they have two polar ends and a non-polar center. One end of a membrane protein contacts the watery extracellular fluid and the other end extends to the watery cytoplasm. The non-polar center remains inside the membrane because it is hydrophobic.
Protein channels facilitate the passage of polar molecules across cellular membranes because the polar amino acids line the inside of the channel and non-polar amino acids line the outside.
The polarity of R groups plays a role in the tertiary structure of globular proteins. Thus, polarity plays a role in shaping enzymes and their active sites.

Muscle contraction (Actin and Myosin)
Transport of other substances (e.g. Haemoglobin)
Membrane Proteins (e.g. Glycoproteins, channel proteins)
Hormones (e.g. Insulin)
Enzymes (e.g. Catalase, Lactase, Amylase)