Protein

• Proteins are large polymers of amino acids
• Proteins contains:

  • carbon
  • hydrogen
  • oxygen
  • nitrogen
  • sulfur
• State the functions of proteins

  • structural support
  • enzymes
  • immunology
  • signalling and transporting
• Proteins are used for structural support- they are the main component of body tissues
• Enzymes are made from proteins, catalysing reactions
• Proteins are used in immunology: antibodies are made from proteins
• Proteins are used for signalling and transporting: hormones and receptors are made from proteins
• There are 20 amino acids
• Monomers for proteins are amino acids
• An amino acid consists of:

  • amino group
  • R group
  • carboxyl group
• Describe the bonds in carboxylic acid (carboxyl group):

  Double bond between carbon and oxygen, single bond between carbon and the OH group
• The R group differentiates amino acids and determines their properties, like if they are hydrophilic or hydrophobic
• Amino acids mostly ends with -ine
• Name three examples of amino acids:

  • alanine
  • glycine
  • cysteine
• The R group for alanine is CH3
• The R group for glycine is H
• The R group for cysteine is CH3S
• Essential amino acids: ingested amino acids obtained from diet that cannot be synthesised in animals
• Plants synthesise all the amino acids they need if they can access fixed nitrogen (nitrates)
• Animals cannot keep excess amino acids in their bodies or the amino group will become toxic, ∴ they are excreted in urine
• Amino acids are joined together by peptide bonds
• Dipeptide molecule: two amino acids joined by peptide bonds
• Polypeptide: longer chains of amino acids
• State the levels of protein structure:

  • primary structure
  • secondary structure
  • tertiary structure
  • quaternary structure
• Describe the primary structure of protein:

  The sequence of amino acids

  • always has an amino group and a carboxyl end
  • changing the order of amino acids in the sequence changes its function
  • determines the shape of the molecule
• Describe the secondary structure of protein:

  when hydrogen bonds between hydrogen and oxygen causes folding called alpha helix or beta pleated

  alpha helix:

  • coils with 36 amino acids per 10 turns of the helix
  • held together by hydrogen bonds between the amino group of an amino acid and the carboxyl group of another four places ahed of its chain

  beta pleated:

  • zigzag structure
  • held together by hydrogen bonds between the amino group and the carboxyl group of another down
• Describe the tertiary structure of protein:

  the overall 3D shape of a protein molecule

  • depends on how the secondary structure of a polypeptide chain folds itself
  • held firmly by bonds between amino acids, e.g. ionic, disulfide, hydrophilic and hydrophobic interactions
• Disulfide bonds happen between the R group of two cysteines as they contain sulfur
• Hydrophobic and hydrophilic interactions:

  hydrophobic R groups arrange themselves in the centre of the polypeptide to avoid water

  hydrophilic R groups arrange themselves at the edge of the polypeptide to be close to water

  this changes the shape of the protein
• Describe the quaternary structure of protein:

  multiple polypeptide chains bonded together to make the complete protein molecule

  e.g. haemoglobin, insulin, collagen, elastin, keratin
• State the two types of protein:

  • globular
  • fibrous
• Describe globular proteins:

  • amino acids arranged in a 3D spherical form
  • soluble in water ∴ hydrophilic R groups are faced outwards
  • complementary to other molecules
  • can contain a prosthetic group / have a co-enzyme
  • affected by temperature and pH
  • Function: catalyse reactions due to its specific shape
• Describe fibrous proteins:

  • regular repetitive sequences of amino acids in long thin structures
  • insoluble in water ∴ hydrophobic R groups are faced outwards
  • strong
  • unreactive
  • Function: structural - elasticity/flexibility
• State three fibrous proteins:

  • collagen
  • keratin
  • elastin
• State three globular proteins:

  • haemoglobin
  • insulin
  • pepsin
• Describe keratin:

  
  

  structure:

  • fibrous
  • quaternary structure
  • rich in cysteine ∴ many disulfide bonds and hydrogen bonds

  Properties and functions:

  • very strong and hard
  • mechanical protection- waterproof, impermeable barrier to infection (waterborne pollutants)
  • Found: fingernails, hair, claws, horns, fur, feathers
• Describe elastin:

  structure:

  • fibrous
  • quaternary structure
  • has cross linking with other peptide chains, can coil

  Properties and function:

  • strong
  • high elasticity- stretchy, adaptable shape
  • Found: skin, lungs, bladder, blood vessels
• Describe pepsin:

  an enzyme that breaks down proteins in the stomach (pH 2)

  • globular
  • tertiary
  • a single polypeptide chain of 327 amino acids that folds into a symmetrical tertiary structure held by hydrogen bonds and two disulfide bonds
  • four amino acids with basic base R groups
  • 43 amino acids with acidic R groups ∴ stable in acidic conditions
• Describe collagen:

  • fibrous
  • quaternary structure
  • three polypeptide chains held by hydrogen bonds
  • covalent bonds form collagen fibril
  • strong but flexible, however cannot stretch
  • found: bones, tendons, artery walls
• Describe insulin:

  • globular
  • quaternary
  • two polypeptide chains: A- starts with a section of alpha helix, B- ends with a section of beta pleated
  • A & B folds into a tertiary structure held by disulfide bonds
  • insoluble - hydrophobic R groups are faced outwards
  • specific fixed shape
  • binds to glycoprotein receptors on muscles and fat cells to increase the uptake of glucose from the blood