Proteins

• Ribosomes are the cellular structures responsible for protein synthesis.
• Amino acids are the monomers of all proteins, and all amino acids have the same basic structure.
• A peptide bond is a bond formed when two amino acids are joined by a condensation reaction.
• The name of almost all amino acids ends in -ine, with exceptions being those which have an acidic R group, such as aspartic acid and glutamic acid.
• R groups can vary by size, by charge and by polarity, with some being hydrophobic and some being hydrophilic.
• Proteins are large polymers comprised of long chains of amino acids.
• The properties of proteins give them a variety of functions: they form structural components of animals in particular, are important as enzymes, antibodies and some hormones, and their tendency to adopt specific shapes makes them important as membrane constituents.
• Membranes have protein constituents that act as carriers and pores for active transport across the membrane and facilitated diffusion.
• Both plants and animals need amino acids to make proteins.
• Animals can make some amino acids, but must ingest the others (called essential amino acids)
• Plants can make all the amino acids they need, but only if they can access fixed nitrogen (such as nitrate).
• Each amino acid contains the elements carbon, hydrogen, oxygen and nitrogen.
• Some amino acids contain sulfur.
• There are over 500 different amino acids, but only 20 of them are proteinogenic, which means that they are found in proteins.
• Each protein chain of amino acids has an amino group (-NH,) at one end, and a carboxyl (-COOH) group at the other end.
• Amino acids are joined together by covalent bonds called peptide bonds.
• Just like the glycosidic bond and the ester bond, making a peptide bond involves a condensation reaction, and breaking a peptide bond involves hydrolysis.
• Enzymes catalyse these reactions.
• Protease enzymes in the intestines break peptide bonds during digestion.
• They also break down protein hormones so that their effects are not permanent.
• All amino acids join together in the same way, whatever R group they may have.
• Two amino acids joined together are known as a dipeptide.
• Joining a longer chain of amino acids together forms a polypeptide.
• A protein may consist of a single polypeptide chain, or more than one chain bonded together.
• The sequence of amino acids in a protein chain is called its primary structure.
• The number and order of amino acids in a protein chain is important, as changing just one amino acid can alter the function of the protein.
• Because there are 20 amino acids, at every point in the chain there are 20 alternatives.
• The function of a protein is determined by its structure.
• Keratin provides mechanical protection, an impermeable barrier to infection, and prevents entry of water-borne pollutants.
• Comparative protein modelling is a technique where a protein threading scan is performed against a database of solved structures to produce a set of possible models which will match the sequence.
• The shape of the haemoglobin molecule is held together by the bonds described in the previous topic.
• Both chains in the insulin molecule fold into a tertiary structure, and are then joined together by disulfide links.
• The quaternary structure of haemoglobin is made up of four polypeptides: two a-globin chains and two 3-globin chains.
• Elastin helps our blood vessels to stretch and recoil as blood is pumped through them, helping maintain the pressure wave of blood as it passes through.
• Keratin is found wherever a body part needs to be hard and strong, including finger nails, hair, claws, hoofs, horns, scales, fur and feathers.
• Pepsin is an enzyme that digests protein in the stomach, the enzyme is made up of a single polypeptide chain of 327 amino acids, but it folds into a symmetrical tertiary structure.
• At one position on the outside of each chain in the haemoglobin molecule, there is a space in which a haem group is held, these groups are called prosthetic groups.
• Haemoglobin carries oxygen from the lungs to the tissues, and in the lungs an oxygen molecule binds to the iron in each of the four haem groups in the haemoglobin molecule.
• Elastin is found in living things where they need to stretch or adapt their shape as part of life processes, such as skin stretching around bones and muscles, and in the lungs and bladder.
• Insulin is made of two polypeptides chains, the A chain begins with a section of a-helix, and the B chain ends with a section of 3-pleat.