Proteins and Enzymes

• Proteins include a diversity of

  structures, resulting in a wide range of

  functions.
• Proteins account for more than 50% of the dry mass of most cells.
• Protein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances.
• Enzymatic proteins

                 

  Function: Selective acceleration of chemical reactions

  Example: Digestive enzymes catalyze the hydrolysis of bonds in food molecules.
•  Storage proteins

                 

  Function: Storage of amino acids

  Examples: Casein, the protein of milk, is the major source of amino acids for baby mammals. Plants have storage proteins in their seeds. Ovalbumin is the protein of egg white, used as an amino acid source for the developing embryo.
• Hormonal proteins

                 

  Function: Coordination of an organism’s activities

  Example: Insulin, a hormone secreted by the pancreas, causes other tissues to take up glucose, thus regulating blood sugar concentration.
•   Contractile and motor proteins

                 

  Function: Movement

  Examples: Motor proteins are responsible for the undulations- in simpler words, vibrations- of cilia and flagella. Actin and myosin proteins are responsible for the contraction of muscles.
• Defensive proteins

                 

  Function: Protection against disease

  Example: Antibodies inactivate and help destroy viruses and bacteria.
• Transport proteins

                 

  Function: Transport of substances

  Examples: Hemoglobin, the iron-containing protein of vertebrate blood, transports oxygen from the lungs to other parts of the body. Other proteins transport molecules across cell membranes.
• Receptor proteins

                 

  Function: Response of cell to chemical stimuli

  Example: Receptors built into the membrane of a nerve cell detect signaling molecules released by other nerve cells.
• Structural proteins

                 

  Function: Support

  Examples: Keratin is the protein of hair, horns, feathers, and other skin appendages. Insects and spiders use silk fibers to make their cocoons and webs, respectively. Collagen and elastin proteins provide a fibrous framework in animal connective tissues.
• Enzymes are a type of protein that acts as a catalyst to speed up chemical reactions
• Enzymes can perform their function repeatedly, functioning as workhorses that carry out the processes of life
• Polypeptides are unbranched polymers built from the same set of 20 amino acids
• A protein is a biologically functional molecule that consists of one or more polypeptides
• Amino acids are organic molecules with carboxyl and amino groups
• Amino acids differ in their properties due to differing side chains, called R groups
• Amino acids are linked by peptide bonds
• A polypeptide is a polymer of amino acids
• Polypeptides range in length from a few to more than a thousand monomers
• Each polypeptide has a unique linear sequence of amino acids, with a carboxyl end (C-terminus) and an amino end (N-terminus)
• The primary structure of a protein is its unique sequence of amino acids is called the primary structure
• Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain that form a helix or a beta-pleated sheet
• Tertiary structure is determined by interactions among various side chains (R groups) are important in determining the biological activity of a drug
• Quaternary structure results when a protein consists of multiple polypeptide chains held together by covalent bonds between the phosphate groups and the sugar
• Primary structure is determined by inherited genetic information
• Primary structure, the sequence of amino acids in a protein, is like the order of letters in a long word
• The coils and folds of secondary structure result from hydrogen bonds between repeating constituents of the polypeptide backbone
• Tertiary structure is determined by interactions between R groups, rather than interactions between backbone constituents
• Strong covalent bonds called disulfide bridges may reinforce the protein’s structure
• Quaternary structure results when two or more polypeptide chains form one macromolecule
• Collagen is a fibrous protein consisting of three polypeptides coiled like a rope
• Hemoglobin is a globular protein consisting of four polypeptides: two alpha and two beta chains
• A slight change in primary structure can affect a protein’s structure and ability to function
• Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin
• What Determines Protein Structure?

       • In addition to primary structure, physical and chemical conditions can affect structure

       • Alterations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel
• This loss of a protein’s native structure is called denaturation
• A denatured protein is biologically inactive
• Chaperonins are protein molecules that assist the proper folding of other proteins
• Diseases such as Alzheimer’s, Parkinson’s, and mad cow disease are associated with misfolded proteins