Amino Acids and Proteins - PiratePanel

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Amino Acids and Proteins Amino Acids and Proteins
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    03-Feb-2022
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Transcript of Amino Acids and Proteins - PiratePanel

AminoAcids• Proteins are composed of amino acids.
• There are 20 amino acids commonly found in proteins. All have:
Cαα
H
COOHNH2
R
Amino acids at neutral pH are dipolar ions (zwitterions) because their αα-carboxyl and αα-amino groups are ionized.
C COONH3
22
44
66
88
pK1pK1
pK2pK2
pHpH COOH= COO- COOH= COO-
NH3+
NH2
22
44
66
88
pK1pK1
pK2pK2
22
44
66
88
pK1pK1
pK2pK2
pKa Values of the Amino Acids
You should know these numbers and know what they mean!
• Alpha carboxyl group - pKa = 2
• Alpha amino group - pKa = 9
• These numbers are approximate, but entirely suitable for our purposes.
COO-
C H2
- C -H3 +N H
COO-
COO-
CH2 CH2
Serine and Threonine can be PHOSPHORYLATED:Serine and Threonine can be PHOSPHORYLATED:
ATP ADP, Pi
ATP ADP, Pi
CH2CH2
SS
Disulfide Bond:Two cysteine residues condense. Disulfide bonds may occur between cyteine residues within the same protein (intrachain) or between two cysteine residues occuring in different proteins (interchain). Disulfide formation is a major factor in the determination of protein structure.
Permanent waving is the result of the reduction of disulfides in the αα-keratin protein (that hair is made of) and spontaneous re-oxidation of those disulfide bonds in air.
Cystine
Titration of Glutamic Acid
Titration of Lysine
A Sample Calculation
What is the pH of a glutamic acid solution if the alpha carboxyl is 1/4 dissociated?
• pH = 2 + log10 [1]
Another Sample Calculation
What is the pH of a lysine solution if the side chain amino group is 3/4 dissociated?
• pH = 10.5 + log10 [3]
• Amino groups form Schiff bases and amides
• Side chains show unique reactivities – Cys residues can form disulfides and can be
easily alkylated
– Few reactions are specific to a single kind of side chain
Stereochemistry of Amino Acids
• D,L-nomenclature is based on D- and L- glyceraldehyde
• R,S-nomenclature system is superior, since amino acids like isoleucine and threonine (with two chiral centers) can be named unambiguously
Spectroscopic Properties
• All amino acids absorb in infrared region
• Only Phe, Tyr, and Trp absorb UV
• Absorbance at 280 nm is a good diagnostic device for amino acids
• NMR spectra are characteristic of each residue in a protein, and high resolution NMR measurements can be used to elucidate three-dimensional structures of proteins
Separation of Amino Acids
• Mikhail Tswett, a Russian botanist, first separated colorful plant pigments by ‘chromatography’
• Many chromatographic methods exist for separation of amino acid mixtures – Ion exchange chromatography
– High-performance liquid chromatography