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Proteins are built from 20 kinds of alpha-amino
acids.
21st aa selenocysteine
CO-R-N clockwise When viewing down from H to
C-alpha ? L-amino acid
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Free -SH group of cysteine -Reacts readily with
heavy metals -Easily oxidized
S
S
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pKa
Three- and one-letter codes for amino acids
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Disulfide bond
Insulin sequence
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Modified amino acid residues
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Rotation around this C-N bond is not free.

• Peptide unit
• -Rigid
• Usually trans
• Dipole moment

H
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These two bonds are free to rotate.
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0.5
0.5-
Alpha-helix has a large dipole moment.
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Antiparallel beta-sheet
Proteins -Fibrous proteins eg. keratin,
collagen, silk -Globular proteins
Collagen sequence
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Collagen triple helix
G, Gly
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Myoglobin is rich in alpha-helices.
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Ribonuclease beta and alpha 4 disulfide bonds
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Poliovirus 60-mer
Hemoglobin tetramer
Beta-alpha-beta motif The crossover connection
is nearly always right-handed.
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Antibody light chain beta-sandwich
Ribonuclease sequence
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Refolding of proteins is very difficult.
Proteins can be easily denatured.
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Conformational change is important for function.