Leucine

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Standard codons for L : CTA CTC CTG CTT TTA TTG

Substitution preferences:
All protein types:
Favoured Ile ( 2) Met ( 2) Val ( 1)
Neutral Phe ( 0)
Disfavoured Ala (-1) Thr (-1) Cys (-1) Tyr (-1) Ser (-2) Arg (-2) Trp (-2) Lys (-2)
Gln (-2) His (-3) Pro (-3) Asn (-3) Glu (-3) Asp (-4) Gly (-4)

Intracellular proteins:
Favoured Ile ( 2) Met ( 2) Val ( 1) Phe ( 1)
Neutral Ala ( 0) Cys ( 0) Trp ( 0) Tyr ( 0)
Disfavoured Thr (-1) Arg (-1) Lys (-1) Gln (-1) His (-1) Pro (-2) Asn (-2) Ser (-2)
Glu (-2) Gly (-3) Asp (-3)

Extracellular proteins:
Favoured Val ( 1) Met ( 1) Ile ( 1)
Neutral Ala ( 0) Pro ( 0) Phe ( 0) Thr ( 0)
Disfavoured Ser (-1) Tyr (-1) Glu (-1) His (-1) Lys (-1) Gln (-1) Arg (-1) Asn (-2)
Gly (-2) Asp (-2) Trp (-2) Cys (-5)

Membrane proteins:
Favoured Met ( 1) Ile ( 1) Phe ( 1)
Neutral Val ( 0)
Disfavoured Thr (-1) Pro (-1) Cys (-1) Gln (-2) Ala (-2) Trp (-2) Ser (-2) Tyr (-3)
Arg (-3) Gly (-4) Lys (-4) His (-4) Asn (-4) Asp (-5) Glu (-5)

Substitutions: As Leucine is an aliphatic, hydrophobic, amino acid, it prefers substitution with other amino acids of the same type (see above).

Role in structure: Being hydrophobic, Leucine prefers to be buried in protein hydrophobic cores. It also shows a preference for being within alpha helices more so than in beta strands.

Role in function: The Leucine side chain is very non-reactive, and is thus rarely directly involved in protein function, though it can play a role in substrate recognition. In particular, hydrophobic amino acids can be involved in binding/recognition of hydrophobic ligands such as lipids.

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Please cite: M.J. Betts, R.B. Russell. Amino acid properties and consequences of subsitutions.
In Bioinformatics for Geneticists, M.R. Barnes, I.C. Gray eds, Wiley, 2003..