CH2 H N C OH O Amino Acids and Dipeptides H N C O R1 OH R2

Polymer: large molecules consisting of large numbers of repeating units connected by covalent bonds Monomer: a small molecule that may become chemically bonded to other monomers to form a polymer amino acid = monomer polypeptide = polymer

General structure of an amino acid R group H N C OH O R 2.2.2 carboxyl group amino group (acidic) (basic)

Different amino acids have different R groups Their different properties depend on their R groups Hydrophobic (‘Water-hating’) R groups CH2 H N C OH O phenylalanine (aromatic) CH3 H N C OH O alanine Do not need to know names or structure of amino acids – interest only

Hydrophillic (‘Water-loving’) R groups CH2 H N C OH O SH cysteine CH2 H N C OH O serine

First isolated from asparagus juice! CH2 H N C OH O CH2 H N C OH O NH2 asparagine (basic) aspartic acid (acidic) First isolated from asparagus juice!

There are 20 amino acids naturally incorporated into proteins Alanine Arginine Asparagine Aspartic acid Cysteine Glutamic acid Glutamine Glycine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Proline Serine Threonine Tryptophan Tyrosine Valine You do NOT need to know their names!

Other amino acids do exist… CH2 H N C OH O I Thyroxine: Other amino acids do exist… used in the human body as a hormone to regulate metabolic rate Some other amino acids do occur in proteins e.g. selenocysteine (some enzymes), hydroxyproline (in collagen) – these are produced by modification of other amino acids after incorporation into proteins. Other amino acids occur as intermediaries in metabolic pathways (ornithine – urea production). D-alanine, D-glutamic acid, mesodiaminopimelic acid are found in the peptide cross-links in bacterial peptidoglycan: only L-forms are found in proteins. Also dopamine (neurotransmitter), creatine (energy store in muscles) …but are not normally found in proteins

These are the essential amino acids Humans can synthesis some amino acids by altering other amino acids, however… …some amino acids cannot be synthesised and are therefore needed in the diet. These are the essential amino acids Arginine Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Interest only but see Option A 2.8

No meat, fish, poultry, eggs, cheese, milk, beans, or peas Phenylketonuria or Maple Syrup Urine Disease Patients lack the enzyme phenylalanine hydroxylase – needed to dispose of the amino acid phenylalanine properly Penylalanine rapidly builds up in the blood stream - it converted to unusual metabolites which give the patient’s urine a characteristic smell Interest only: students may have seen ‘contains a source of phenylalanine’ on food packaging - it also rapidly causes brain damage! Treatment: A diet low in phelylalanine No meat, fish, poultry, eggs, cheese, milk, beans, or peas

Formation of a dipeptide: carboxyl group of one amino acid reacts with amino group of second amino acid H N C OH O R2 R1 water molecule formed H N C O R1 OH R2 H2O 2.2.6 a dipeptide peptide linkage

2 amino acids  dipeptide + water Formation of a dipeptide produces a molecule of water, therefore… H N C OH O R1 R2 H2O …this is a condensation reaction H N C O R1 OH R2 Part of 2.2.5 2 amino acids  dipeptide + water

dipeptide + water  2 amino acids Splitting a dipeptide to form two amino acids consumes one molecule of water, therefore… H N C O R1 OH R2 H2O H N C OH O R1 R2 …this is a hydrolysis reaction Part of 2.2.5 dipeptide + water  2 amino acids

three amino acids linked together Tripeptide: three amino acids linked together Oligopeptide: a short chain of a few amino acids Polypeptide: a chain of many amino acids A protein may consist of one or more polypeptide chains Structure of hemoglobin – a protein consisting of four polypeptides