Proteins

• The primary structure of a protein is the sequence of amino acids.
• Proteins account for more than 50% of the dry mass of most cells
• Humans have tens of thousands of different proteins, each with their own structure and function
• Proteins are the most structurally complex molecules known
• Each type of protein has a complex 3-dimensional shape or conformation
• Proteins are instrumental in organisms
• Enzymatic proteins: Enzymes in a cell that regulate metabolism by selectively accelerating chemical reactions
• Structural proteins: Provide structural support, e.g. Keratins strengthen protective coverings such as hair, quills, feathers, horns, and beaks
• Transport proteins: Carrier proteins that transport substances around the body, e.g. haemoglobin carries oxygen from the lungs to other parts of the body
• Defensive proteins: Protect or defend against foreign substances, e.g. antibodies provide immunities against infection by bacteria or viruses
• Storage proteins: Store molecules, e.g. ovalbumin the protein of egg white for developing embryo, casein the protein in milk for baby mammals
• Contractile and motor proteins: Involved in movement, e.g. actin & myosin are responsible for the movement of muscles
• Receptor proteins: Involved in intercellular signaling and cellular communications, e.g. membrane of a nerve cell have protein receptors which can detect chemical signals released by other nerve cells
• Amino acids consist of four components attached to a central carbon, the alpha carbon
• Differences in R groups produce the 20 different amino acids (monomers)
• Amino acids are joined together when a dehydration reaction removes a hydroxyl group from the carboxyl end of one amino acid and a hydrogen from the amino group of another, resulting in a peptide bond
• A polypeptide is a polymer of amino acids constructed from the same set of 20 amino acids
• Each polypeptide has a unique linear sequence of amino acids; at one end is an amino acid with a free amino group (N-terminus) and at the other is an amino acid with a free carboxyl group (C-terminus)
• Protein structure has 4 levels: primary, secondary, tertiary, and quaternary structure
• Primary structure refers to the unique sequence of amino acids in a polypeptide chain
• Secondary structure results from the twisting of the primary structure in the form of coils (α-helix) or folds (β-pleated sheets)
• Tertiary structure is the result of folding of the secondary structure in space, determined by interactions among R groups and between R groups and the polypeptide backbone
• Quaternary structure results from the aggregation of two or more polypeptide chains into a macromolecule
• Alterations in pH, salt concentration, temperature, or other factors can denature a protein, disrupting its shape and biological activity
• Proteins require assistance from chaperone proteins or Chaperonins to convert a linear chain of amino acids to a functional 3D unit
• Cystic fibrosis is a hereditary disorder in which a mutation disables a protein that plays a vital part in moving ions across cell membranes, leading to protein misfolding and disease