Protein

• A diversity of structures, a wide range of functions • Used in almost everything we do • Make up about 50% of cell dry mass • Important role of proteins – Enzymes • A chemical agent that increases the rate of a reaction without being consumed by the reaction – By doing this, enzymes regulate metabolism

Protein functions

• Enzymatic proteins → Selective acceleration of chemical reactions. Ex: Digestive enzymes catalyze the hydrolysis of bonds in food molecules
• Defensive proteins → Protect against disease. Ex: Antibodies inactivate and help destroy viruses and bacteria
• Storage proteins → Storage of amino acids. Ex: Plants have storage protein in their seeds.
• Transport proteins → Transport of substances. Ex: Hemoglobin, the iron-containing protein of vertebrate blood, transports oxygen from the lungs to other parts of the body.
• Hormonal proteins → coordination of an organism’s activities. Ex: Insulin, a hormone secreted by the pancreas, causes other tissues to take up glucose, thus regulating blood sugar concentration.
• Receptor proteins → response of cell to chemical stimuli. Ex: Receptors built into the membrane of a nerve cell detect signalling molecules released by other nerve cells.
• Contractile and motor proteins → movement. Ex: Contraction of muscles Structural proteins → support. Ex: hair, horn, feathers

• Monomers of proteins = Amino acids • Polymers (made up of several amino acids) → Peptides or polypeptides → Unbranched → Made from set of 20 amino acids • A protein is a biologically functional molecule made up of one or more polypeptides, each folded and coiled into a specific 3-D structure

AMINO ACID: Centre is asymmetric C atom called alpha C – NH3+ group – COO- group – H atom – R or variable group

• Variable group, R – R is the side chain, varies with each amino acid – Side chain determines characteristics of the amino acid

• Amino acid polymers – Linked to form chains by dehydration reactions • Resulting covalent bond is a peptide bond

• Polypeptides – A few amino acids to over 1000 – Each specific polypeptide has a unique linear sequence of amino acids • Polypeptides have an N terminus and a C terminus Structure and function • Activities due to specific 3-D structure • Determined by amino acid sequence

• How do you go from chain to functional protein? • Chain may fold on its own, bonds formed depends on the sequence of amino acids • Roughly spherical = globular proteins • Long fibres = fibrous proteins

• Function often depends on the ability to recognize and bind to some other molecule • Examples – Antibodies are proteins with specific shapes – Morphine, heroin and other opiates mimic endorphins and so can bind to receptors (have similar shape) • Lock-and-key specificity → Sickle-cell disease – Caused by substitution of one amino acid for another at a particular place in the sequence of hemoglobin (protein that carries oxygen in red blood cells) – These aggregate, get deformed into sickle shape, sometimes cause crisis as they clog small blood vessels – Misfolding of polypeptides associated with certain diseases → Cystic fibrosis, Alzheimer’s, Parkinson’s, mad cow disease