Unit 3: Protein Shape in Relation to Function

Channel Proteins

Proteins that fold to form channels in the plasma membrane present non-polar R groups to the inside the channel.

Hydrophilic molecules and ions are able to pass through these channels into the interior of the cell. Iron channels are found in nearly all cells and many organelles.

Has both alpha helix and beta pleated sheets.

Enzymes

Enzymes are globular proteins that catalyze reactions. They are specific to their substrates and their tertiary structure creates an active site where the substrate can bring and the reaction can occur.

The specificity of the active site is determined by the interactions of the amino acid R groups.

Denaturation alters the active site and causes a loss of function.

Sub-unit proteins

many protein, e.g. insulin and hemoglobin, consist of two or more sub-units in a complex quaternary structure, often in associate with a metal ion.

Insulin

Active insulin is formed by two polypeptide chains stabilized by disulfide bridges between neighboring cysteines. Insulin simulates glucose uptake by cells.

Protein denaturation

When the chemical bonds holding a protein together become broken the protein can no longer hold its three dimensional shape. The protein usually loses its ability to carry out its biological function.

ex:
The main protein in egg white is albumin. It has a clear, thick fluid appearance in a raw egg. Head (cooking) denatures the albumin protein and it becomes insoluble, clumping together to form a thick white substance.

Causes of denaturation

- exposure to heat
- pH outside of the proteins optimum range