Chapter 26: Biomolecules: Amino Acids Peptides and Proteins
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Transcript of Chapter 26: Biomolecules: Amino Acids Peptides and Proteins
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Chapter 26: Biomolecules: Amino Acids Peptides and Proteins
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Chirality of Amino Acids
• Glycine, 2-amino-acetic acid, is achiral• In all the others, the carbons of the amino acids are centers
of chirality• The stereochemical reference for amino acids is the Fischer
projection of L-serine• Proteins are derived exclusively from L-amino acids
D-glyceraldehyde L-glyceraldehyde
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Draw Fisher diagrams of L-Alanine (R = CH3) and L-cysteine (R = CH2-SH) and assign stereochemistry as R or S
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Amino Acids: pKa and Isoelectric points
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26.2 Isoelectric Points
• In acidic solution, the carboxylate and amine are in their conjugate acid forms, an overall cation
• In basic solution, the groups are in their base forms, an overall anion
• In neutral solution cation and anion forms are present
• This pH where the overall charge is 0 is the isoelectric point, pI
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Titration Curves of Amino Acids
• pKa’s determined from titration curve
• If pKa values for an amino acid are known the fractions of each protonation state can be calculated (Henderson-Hasselbach Equation)
• pH = pKa – log [A-]/[HA]
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pI of any amino acid is the average of the two pKa’s that involve the neutral zwitterion
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pI of any amino acid is the average of the two pKa’s that involve the neutral zwitterion
pI of Histidine ?(pKa’s = 1.82, 9.17, 6.00)
Charge of Histidine at pH = 7.0?
• Contains an imidazole ring that is partially protonated in neutral solution
• Only the pyridine-like, doubly bonded nitrogen in histidine is basic.
• The pyrrole-like singly bonded nitrogen is nonbasic because its lone pair of electrons is part of the 6 electron aromatic imidazole ring (see Section 24.4).
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Peptides (listed from N to C terminus)
Draw structure of Ala-Ser-Asp. What is the net charge of the peptide at pH 7.5?Ala, R = CH3; Ser, R = CH2OH; Asp (R = CH2CO2H
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26.10 Peptide Synthesis
• Peptide synthesis requires that different amide bonds must be formed in a desired sequence
• The growing chain is protected at the carboxyl terminal and added amino acids are N-protected
• After peptide bond formation, N-protection is removed
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Carboxyl Protecting Groups
• Usually converted into methyl or benzyl esters
• Removed by mild hydrolysis with aqueous NaOH
• Benzyl esters are cleaved by catalytic hydrogenolysis of the weak benzylic C–O bond
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Amino Group Protection
• An amide that is less stable than the protein amide is formed and then cleaved after amide bond formed
• The tert-butoxycarbonyl amide (BOC) protecting group is introduced with di-tert-butyl dicarbonate
• Removed by brief treatment with trifluoroacetic acid
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Peptide Coupling
• Amides are formed by treating a mixture of an acid and amine with dicyclohexylcarbodiimide (DCC)
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26.11 Automated Peptide Synthesis: The Merrifield Solid-Phase Technique
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26.13 Protein Structure
• The primary structure of a protein is simply the amino acid sequence.
• The secondary structure of a protein describes how segments of the peptide backbone orient into a regular pattern.
• The tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape.
• The quaternary structure describes how different protein molecules come together to yield large aggregate structures