Shelly

• Proteins


  Diverse group of molecules that can exist in many different shapes and sizes, can exhibit a range of chemical and physical properties
• Classes of proteins


  • Fibrous proteins
  • Globular proteins
• Fibrous proteins


  • Important structural proteins, long, thread-like, tough and insoluble in water
  • Include collagens, keratin, fibrin, elastins, muscle proteins (myosins, actin), fibroin
  • Found in bone, skin, tendons, cartilage, hair, wool, fur, nails, claws, scales, feathers, muscle, silk
• Collagens


  • 25-35% total protein in mammals
  • Major component of connective tissues (bones, teeth, cartilage, tendons, ligaments, blood vessels, skin)
  • In bone, found along with calcium phosphate polymer
  • In tendons, collagen forms stiff fibres with high tensile strength
  • In skin, collagen is loosely woven allowing expansion in different directions
• Keratin structure


  • Major component of skin, fur, hair, wool, claws, nails, hooves, horns, scales, beaks, feathers
  • Composed almost entirely of helix structures (slightly more tightly wound than α-helix)
  • The basic unit of structure is the protofibril
• Globular proteins


  • Have important 'biological' or 'operational' functions
  • Roughly spherical in shape
  • More soluble than fibrous proteins
  • Include enzymes, some hormones (peptide hormones), haemoglobin, myoglobin, albumin, antibodies
• Conjugated proteins

  • Lipoproteins
  • Glycoproteins
  • Nucleoproteins
  • Metalloproteins
• Amino acids

  The building blocks of proteins, all proteins are made from the same 20 amino acids, different amino acids have different chemical properties
• Amino acid side chains

  • Vary in size, shape, charge, chemical reactivity, ability to form chemical bonds, solubility in water
  • This means proteins can assume thousands of 3-D shapes and have so many roles
• Classes of amino acids

  • Aliphatic
  • Imino
  • Hydroxy
  • Sulphur-containing
  • Aromatic
  • Heterocyclic
  • Basic
  • Acidic (+ amide derivatives)
• Aliphatic amino acids


  • C atoms (in side chain) are arranged in chains
  • Because they only contain C & H (similar electronegativities), they prefer to make non-polar covalent bonds
  • This means they are only partially soluble in H2O (weakly hydrophobic)
  • The longer the side chain, the more hydrophobic the amino acid
• Hydroxy amino acids


  • Have side chains containing a hydroxyl (OH) group
  • Because these side chains contain O as well as C & H (different electronegativities), they prefer to make polar bonds
  • This means they are readily soluble in H2O (hydrophilic)
• Sulphur-containing amino acids


  • Have aliphatic side chains with thiol (SH) or thioether (S-CH3) groups
  • Are hydrophobic amino acids
• Aromatic amino acids


  • Side chains contain benzene skeleton
  • Are hydrophobic amino acids
• Heterocyclic amino acids


  • E.g. histidine, the ring structure is not composed of just one sort of atom
  • Histidine is hydrophilic
• Basic amino acids


  • Are positively charged at neutral pH
  • Are hydrophilic
• Acidic amino acids


  • Are negatively charged at neutral pH
  • Have a secondary carboxylic acidic (COO-) group
  • Their amide derivatives have an amide group instead (CONH2)
  • Are hydrophilic
• Imino acid


  • Has an aliphatic side chain, cyclised by bonding to the N atom of the amino group
  • Is hydrophobic
• Amino acid solubility


  • Amino acid side chain plays a large part in controlling its chemical and physical properties
  • Solubility in an aqueous environment depends on the structure of the side chain
  • Some side chains are hydrophobic (aliphatic, imino, sulphur-containing, aromatic), some hydrophilic (hydroxy, heterocyclic, basic, acidic)
  • The degree of solubility dictates if amino acids are embedded within, or found on the surface of, proteins within cells
  • Hydrophobic amino acids are not likely to be found on the surface
  • Regions of proteins containing hydrophobic amino acids may exist in hydrophobic intracellular environments (e.g. within membranes)
• Levels of protein structure

  • Primary structure
  • Secondary structure
  • Tertiary structure
  • Quaternary structure
• Primary structure


  The sequence of amino acids of a polypeptide (and location of disulphide bridges)
• Amino acids are polymerised into peptides and polypeptides


  1. Catalysed by peptidyl transferase enzymes

  2. Requires a template (RNA)

  3. Dehydration reaction (releases H2O)
• Peptide bond


  Formed when the -carbonyl group of the first amino acid becomes covalently linked to the -amino group of the second amino acid, eliminating 1 molecule of water
• Peptide bond formation


  1. Forms a peptide bond between the 2 amino acid residues to form a dipeptide

  2. Addition of a 3rd aa forms a tripeptide

  3. A chain of aa residues linked by peptide bonds is called an oligopeptide (2-10 amino acids) or a polypeptide (>10)
• Cysteine forms disulphide bonds


  2 cysteine residues link together to form a disulphide bond or bridge, the altered structure is called cystine
• Disulphide bonds link distant regions of primary structure together
• Primary structure of bovine ribonuclease is 124 amino acids with 4 disulphide bonds
• A difference in a single amino acid

  Sickle cell anaemia
• Secondary structure


  Different regions within a polypeptide may adopt certain arrangements that stabilise the protein as it assumes its final 3D structure
• Alpha helix


  • Visualised like the inside of a cylinder, invariably right-handed, held together by hydrogen bonds between the carbonyl oxygen and NH groups 4 steps apart
• Proline (imino acid)


  Tends to be a 'helix breaker' as its side chain occupies space that a neighbouring residue would usually occupy
• Beta-pleated sheet


  • Extended polypeptide chains (strands) stabilised by hydrogen bonding with aa in other strands, can link different regions within a single polypeptide or link 2 polypeptides together, hydrogen bonds are almost perpendicular to the extended strands, can be parallel or anti-parallel
• Loops


  Connect regions of repetitive secondary structure, provide changes of direction necessary for the protein to assume its correct shape, often contain hydrophilic amino acid residues and tend to be found on the surface of globular proteins
• Turns


  A loop with only a few amino acid residues, Type I or Type II
• Tertiary structure


  The 3D-structure of an entire globular protein in its biologically active shape, normally stabilised by non-covalent interactions between side chains, compactly folded regions within the tertiary structure are called domains and usually have a specific function
• Sources of lipids in the diet

  • Triglycerides (containing SFAs, TFAs, MUFAs & PUFAs)
  • Cholesterol
  • Phospholipids
• Triglycerides


  Containing saturated fatty acids (SFAs), trans fatty acids (TFAs), monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs)
• Sources of triglycerides


  • Meat, poultry, dairy foods, coconut oil, palm oil
• Sources of trans fatty acids (TFAs)

  • Commercially fried & baked goods, margarine, vegetable shortening
• Sources of monounsaturated fatty acids (MUFAs)

  • Olive, peanut & rapeseed oils; nuts, avocados