Page 32 - 86 human physiology part-2

Nutrition

Behaves similarly to glutamic acid. Carries a hydrophilic acidic group with
strong negative charge. Usually is located on the outer surface of the protein,
Aspartic acid D Asp making it water-soluble. Binds to positively-charged molecules and ions,
often used in enzymes to fix the metal ion. When located inside of the
protein, aspartate and glutamate are usually paired with arginine and lysine.
Behaves similar to aspartic acid. Has longer, slightly more flexible side
Glutamate E Glu
chain.
Essential for humans. Phenylalanine, tyrosine, and tryptophan contain large
rigid aromatic group on the side chain. These are the biggest amino acids.
Phenylalanine F Phe
Like isoleucine, leucine and valine, these are hydrophobic and tend to orient
towards the interior of the folded protein molecule.
Because of the two hydrogen atoms at the α carbon, glycine is not optically
active. It is the smallest amino acid, rotates easily, adds flexibility to the
Glycine G Gly protein chain. It is able to fit into the tightest spaces, e.g., the triple helix of
collagen. As too much flexibility is usually not desired, as a structural
component it is less common than alanine.
In even slightly acidic conditions protonation of the nitrogen occurs,
changing the properties of histidine and the polypeptide as a whole. It is used
by many proteins as a regulatory mechanism, changing the conformation and
Histidine H His
behavior of the polypeptide in acidic regions such as the late endosome or
lysosome, enforcing conformation change in enzymes. However only a few
histidines are needed for this, so it is comparatively scarce.
Essential for humans. Isoleucine, leucine and valine have large aliphatic
hydrophobic side chains. Their molecules are rigid, and their mutual
Isoleucine I Ile
hydrophobic interactions are important for the correct folding of proteins, as
these chains tend to be located inside of the protein molecule.
Essential for humans. Behaves similarly to arginine. Contains a long flexible
side-chain with a positively-charged end. The flexibility of the chain makes
lysine and arginine suitable for binding to molecules with many negative
charges on their surfaces. E.g., DNA-binding proteins have their active
Lysine K Lys
regions rich with arginine and lysine. The strong charge makes these two
amino acids prone to be located on the outer hydrophilic surfaces of the
proteins; when they are found inside, they are usually paired with a
corresponding negatively-charged amino acid, e.g., aspartate or glutamate.
Essential for humans. Behaves similar to isoleucine and valine. See
Leucine L Leu
isoleucine.
Essential for humans. Always the first amino acid to be incorporated into a
protein; sometimes removed after translation. Like cysteine, contains sulfur,
Methionine M Met but with a methyl group instead of hydrogen. This methyl group can be
activated, and is used in many reactions where a new carbon atom is being
added to another molecule.
Similar to aspartic acid. Asn contains an amide group where Asp has a
Asparagine N Asn
carboxyl.
Proline P Pro Contains an unusual ring to the N-end amine group, which forces the CO-
NH amide sequence into a fixed conformation. Can disrupt protein folding
structures like α helix or β sheet, forcing the desired kink in the protein
chain. Common in collagen, where it often undergoes a posttranslational

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