Proteins

• Proteins are polymers made of monomers called amino acids; the sequence, type, and number of amino acids within a protein determine its shape and function
• Proteins are crucial in cells as they form enzymes, cell membrane proteins, hormones, immunoproteins, transport proteins, structural proteins, and contractile proteins
• Amino acids are the monomers of proteins, with 20 amino acids common to all living organisms
• In a condensation reaction to form a peptide bond, a hydroxyl (-OH) is lost from the carboxylic group of one amino acid and a hydrogen atom is lost from the amine group of another amino acid
• Dipeptides are formed by the condensation of two amino acids, while polypeptides are formed by the condensation of many (3 or more) amino acids
• During hydrolysis reactions, water breaks the peptide bonds, resulting in polypeptides being broken down to amino acids
• Chromatography is a technique used to separate mixtures into individual components based on differences in solubility; it involves a mobile phase and a stationary phase
• Paper chromatography, a specific form of chromatography, uses a liquid solvent as the mobile phase and chromatography paper as the stationary phase to separate components in a mixture
• Paper chromatography can be used to separate a mixture of amino acids by comparing their solubility in the mobile phase, allowing for identification of unknown amino acids
• Proteins have four levels of structure, with the primary structure being the sequence of amino acids bonded by covalent peptide bonds, determined by the DNA of a cell
• The primary structure of a protein is specific for each protein, with one alteration in the sequence of amino acids affecting the function of the protein
• The primary structure of a protein uses three-letter abbreviations to indicate the specific amino acid, with 20 commonly found in cells of living organisms
• Secondary structure of a protein occurs when weak negatively charged nitrogen and oxygen atoms interact with weak positively charged hydrogen atoms to form hydrogen bonds
• Two shapes that can form within proteins due to hydrogen bonds are the α-helix and the β-pleated sheet
• The α-helix shape occurs when hydrogen bonds form between every fourth peptide bond, while the β-pleated sheet shape forms when two parts of the polypeptide chain are parallel to each other enabling hydrogen bonds to form between parallel peptide bonds
• Most fibrous proteins have secondary structures like collagen and keratin
• Tertiary structure of a protein involves further conformational change of the secondary structure, leading to additional bonds forming between the R groups (side chains)
• The additional bonds in the tertiary structure of a protein are hydrogen bonds, disulphide bonds (between cysteine amino acids), ionic bonds, and weak hydrophobic interactions
• Quaternary structure occurs in proteins with more than one polypeptide chain working together as a functional macromolecule, for example, haemoglobin
• Each polypeptide chain in the quaternary structure is referred to as a subunit of the protein
• Proteins have interactions between R groups that determine their shape and function, forming the tertiary structure of a protein
• Disulphide bonds are strong covalent bonds that form between two cysteine R groups, helping stabilize proteins and can be broken by reduction
• Ionic bonds form between positively charged (amine group -NH3+) and negatively charged (carboxylic acid -COO-) R groups, being stronger than hydrogen bonds but not common
• Hydrogen bonds form between strongly polar R groups, being the weakest bonds but the most common as they form between a wide variety of R groups
• Hydrophobic interactions form between non-polar (hydrophobic) R groups within the interior of proteins
• Globular proteins play physiological roles like enzymes catalyzing specific reactions and immunoglobulins responding to specific antigens
• Globular proteins have specific shapes due to the folding of the protein resulting from interactions between the R groups
• Some globular proteins are conjugated proteins containing a prosthetic group, e.g., haemoglobin with the prosthetic group called haem
• Fibrous proteins are long strands of polypeptide chains with cross-linkages due to hydrogen bonds, having little or no tertiary structure
• Fibrous proteins, due to their insolubility in water, are suitable for structural roles like keratin in hair, nails, horns, and feathers, and collagen in skin, tendons, and ligaments
• Haemoglobin is a globular protein with a quaternary structure, carrying oxygen in red blood cells with four polypeptide chains held together by disulphide bonds
• The prosthetic haem group in haemoglobin contains an iron II ion (Fe2+) that can reversibly combine with an oxygen molecule, forming oxyhaemoglobin
• Haemoglobin's function is to bind oxygen in the lung and transport it to tissues for aerobic metabolic pathways
• Collagen, the most common structural protein in vertebrates, forms connective tissues like tendons, cartilage, ligaments, bones, and skin
• Collagen is an insoluble fibrous protein formed from three polypeptide chains closely held together by hydrogen bonds to form a triple helix (tropocollagen)
• Collagen's structure includes glycine, proline, and hydroxyproline amino acids, with every third amino acid being glycine in the primary structure