The functions of protein are linked to their shape
Proteins are polymers of amino acids joined by strong peptide bonds. The combination of any of the twenty amino acids in any length and sequence allows an almost infinite number of possible structures and functions.
The sequence of amino acids in the polypeptide chain is termed the primary structure. The primary structure is unique to a given protein. The primary structure can fold regularly to form either an α-helix or β-pleated sheet. The secondary structure is held together by hydrogen bonds between adjacent peptide bonds. The primary structure can further fold in an irregular but not random manner to form an overall three ...view middle of the document...
Some proteins adopt a transport role. Channel proteins in cell membranes offer a hydrophilic passage through the hydrophobic lipid bilayer. They have a specific three dimensional shape that is complementary to the given species they transport. For example, sodium gated channels in membranes of sensory neurones allow the passage of sodium into the axon during the generation of an action potential. Similar transport proteins are carrier proteins that can change shape on binding of their transporter molecule, e.g. glucose channel in liver cells to allow glucose to pass through the membrane in preparation for the process of glycogenesis.
Proteins form a key role in the infectivity of pathogens and the immunity of the host. Proteins on the surface of pathogenic bacteria act as antigens which identify a cell as non-host. Some of these antigens can break away and act as toxins. For example, the bacteria Vibrium cholerae releases a protein toxin that opens chloride ion channels in the large intestine causing loss of chloride from epithelial cells, and loss of large volumes of water as diarrhoea and chronic dehydration. Variation in the antigenic structure brought about by mutation of the pathogen’s DNA can increase the infectivity of the pathogen as the host has no memory cells or antibodies to bind to and inactivate the antigen. Phagocytosis of pathogens eventually leads to activation of B-cells
which divide by mitosis forming clones that differentiate to form plasma cells. These cells release antibodies that are globular proteins which have variable regions that have a complementary shape to a specific antigen, allowing it to agglutinate many pathogenic particles.
A key role for proteins is to act as enzymes; biological catalysts that lower activation energy of specific reactions, allowing them to take place under controlled conditions at body temperature. They have an active site that has a specific 3D shape...