Amino Acid Diversity (HL) (HL IB Biology)

• The same 20 amino acids make up most of the proteins found on Earth
• Around 500 amino acids have been found in nature, but only 20 are commonly found in proteins
• Eleven of these can be naturally synthesised within cells by humans
• The other nine amino acids are essential (have to be in the human diet)
• You don't need to remember the names of the amino acids, but it's useful to see their names, which are usually abbreviated to three letters
  • Ala, Arg, Asn, Asp, Cys, Gln, Glu, Gly, His*, Ile*, Leu*, Lys*, Met*, Phe*, Pro, Ser, Thr*, Trp*, Tyr, Val*
  • *  indicates the essential amino acids
• Because the R groups vary so much between the 20 amino acids, there is a lot of chemical diversity between the amino acids

Amino acid diversity diagram

An amino acid sequence of a short polypeptide. The three-letter abbreviations indicate the specific amino acid (there are 20 commonly found in cells of living organisms).

R-groups

• An R-group, or variable group, is how each amino acid differs and why amino acid properties differ e.g. whether they are acidic or basic, or whether they are polar or non-polar
• The R-group can be as simple as another hydrogen atom (forming the amino acid glycine), right through to complex aromatic ring structures (which forms phenylalanine)
• It is the R-group that gives the basis of diversity in protein function and form
• This occurs because: 
  • All amino acids have a the same carboxyl and amine group, bonded by a central carbon atom, which ionise when in a neutral environment
    • The carboxyl group donates a hydrogen ion whilst the amine group accepts it so that the carboxyl group is left with a negative charge and the amine group left with a positive charge
  • The R-group is either hydrophobic or hydrophilic
    • Hydrophilic R-groups are polar and can be either acidic or basic
    • Hydrophobic R-groups are non-polar

R-Group Diagram

The variety of R-groups of the amino acids determine the properties of polypeptide chains and therefore overall protein