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AMINO ACIDS
Functions of Amino Acids
● A variety of roles in metabolism
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)
● They can have hormonal and catalytic function
● Several genetic disorders are caused in amino acid metabolism errors (aminoaciduria - presence of
amino acids in urine)
Standard and Nonstandard Amino Acids
● More than 300 different amino acids have been described in nature
● Standard α-amino-acids:
○ Only 20 are commonly found as constituents of proteins
● Nonstandard amino acids:
○ Amino acid derivatives found in protein
○ Non-protein amino acid
Amino acids are substituted acids in which an amino group (NH2) has been substituted for a hydrogen
atom attached to the carbon in the alpha position
...
The most
helpful start is to separate amino acids into:
1
...
Neutral polar
3
...

Nonpolar (Hydrophobic) R Groups

The simplest amino acid is Glycine, which has a single hydrogen atom as its side chain
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e
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Leucine and Isoleucine are isomers of each other
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Phenylalanine is highly hydrophobic and is found buried within globular proteins

Phenylalanine
● Tryptophan is highly hydrophobic and tends to be found immersed inside globular proteins
● Structurally related to Alanine, but with a two ring (bicyclic) indole group added in place of the single
aromatic ring found in Phenylalanine
● The presence of the nitrogen group makes Tryptophan a little less hydrophobic than Phenylalanine

Tryptophan
● Proline is unique amongst the amino acids - its side chain is bonded to the backbone nitrogen as
well as to the α-carbon
● Because of this proline is technically an imino rather than an amino acid
● The ring is not reactive, but it does not restrict the geometry of the backbone chain in any protein
where it is present

Proline
Neutral Amino Acids - Polar (Hydrophilic) R Groups

● Tyrosine is Phenylalanine with an extra hydroxyl (-OH) group attached
● It is polar and very weakly acidic
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Reversible phosphorylation of -OH group in some enzymes is important in the
regulation of metabolic pathways
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Serine

Threonine

● Cysteine has sulfur-containing side group
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It is not
very polar

Cysteine
● Cystine is most important for its ability to link to another cysteine via the sulfur atoms to form a
covalent disulfide bridge, important in the formation and maintenance of the tertiary (folded)
structure in many proteins
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They cannot be ionized and are therefore uncharged
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Negatively (Nonpolar) Charged R
Groups

Aspartic Acid (Asp)

Glutamic Acid (Glu)

Basic Amino Acids - More amino group than carboxylic group

Classification Based on Chemical Constitution:
➔ Small Amino Acids - Glycine, Alanine
➔ Branched Amino Acids - Valine, Leucine, Isoleucine
➔ Hydroxy Amino Acids (-OH Group) - Serine, Threonine
➔ Sulfur Amino Acids - Cysteine, Methionine
➔ Aromatic Amino Acids - Phenylalanine, Tyrosine, Tryptophan
➔ Acidic Amino Acids and their Derivatives - Aspartate, Asparagine, Glutamate, Glutamine
➔ Basic Amino Acids - Lysine, Arginine, Histidine
➔ Imino Acid - Proline
➔ Nonpolar Amino Acids:
◆ Alanine (Ala)

Valine (Val)

Leucine (Leu)

Isoleucine

Proline (Pro)

Phenylalanine (Phe)

Tryptophan

(Ile)
◆ Glycine (Gly)
(Trp)
➔ Polar, Uncharged Amino Acids:
◆ Serine (Ser)

Threonine (Thr)

Cysteine (Cys)

◆ Glutamine (Gln)

Methionine (Met)

Tyrosine (Tyr)

➔ Acidic Amino Acids
◆ Aspartic Acid (Asp)

Glutamic Acid (Glu)

➔ Basic Amino Acids
◆ Lysine (Lys)

Arginine (Arg)

Histidine (His)

Asparagine (Asn)

Aromatic Amino Acids
● Phe, Tyr, and Trp
○ Phe and Tyr: Benzene rings
○ Tryptophan: Indole ring

● The -OH group in Tyr is an important functional group in proteins (Phosphorylation, hydrogen bond,
etc,

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