BCHM-3050 Lecture Notes - Lecture 8: Acid Dissociation Constant, Tripeptide, Myoglobin
8 Jun 2018
School
Department
Course
Professor
Peptide bond links amino acids
Peptide bond: formed through the condensation of two amino acids
Eliminate a water molecule to form a new bond b/w carboxylic acid
of one AA and the amino group of another
○
Opposite of hydrolysis
○
Must be a water molecule
○
•
Rxn is not favorable
Couple with ATP hydrolysis during translation
○
Reverse rxn is favorable
○
•
Peptide bond = planar
No motion here
○
•
Other bonds besides peptide bond = cis or trans
Trans = favored
Less steric hindrance
§
○
•
Peptide bond cleavage
The delta G for hydrolysis is favorable
•
Peptides are very unstable
Due to resonance
○
UNLESS! Catalyst or strong acid is present
Need them to break bond
§
○
•
6 N HCL at high T = degrade protein
•
Proteases: enzyme that cleave peptide bonds
Cleavage under gentler conditions
○
•
Cyanogen bromide: cleave amide bonds on C terminal of methionyl residues
•
Edman degradation: peptide could have amino acid sequence determined by
stepwise release
•
Enzymatic proteolysis
Preferred site: what bond will be cut
•
R1 ( N terminus)- BEFORE AA or R2 (C terminus)- AFTER AA
Amino acid cleave differently
○
Ex. Trypsin cleaves R1 site of Lys and Arg
○
•
EGAK tetrapeptide
Oligopeptide: chains with few amino acid residues
•
Polypeptide: a dozen or so residues
•
EGAK oligopeptide = Glu - Gly - ALA - Lys
Tetrapeptide
○
No free rotation
○
•
Peptide and proteins = polyampholytes
As pH increases overall peptide charge = more negative
And vice versa
○
•
Pka of side chain is influenced by environment
Alone glutamic acid = 4.2
Next to negative charge = increase pKA
Makes the amino acid more negative
§
○
Next to positive charge = decrease pKa
Make more positive
§
○
•
Glutathione
Glutathione: tripeptide sequence
Use beta carbon on glutamate to make bond
○
Regulates cell redox state and prevents damage by ROS
○
•
Suck up electrons
Prevent redox state from becoming too oxidized
○
•
Dimerize by forming disulfide bond b/w two Cys residues
•
All occurs when cell is under oxidative stress
•
N and C terminus modified in peptides
Formylation
•
Acetylation: may prevent protein secretion pathway
•
Amidation: C terminal
Add amide
○
Increase half life of hormonal peptide
○
•
Acetyl and amid: prevent formation of charged ends
Help signal to cell which proteins can or cant be secreted on cell
Like tag!
§
○
•
Amino acid sequence of human and whale myoglobin (mutation)
Conservative mutation: keep chemical properties (ex. Charge) and or size of
AA
•
Nonconservative: involve large difference in polarity/size
•
Chapter 5: Introduction to proteins (P2)
Thursday, May 31, 2018
12:00 AM
Document Summary
Peptide bond: formed through the condensation of two amino acids. Eliminate a water molecule to form a new bond b/w carboxylic acid of one aa and the amino group of another. Other bonds besides peptide bond = cis or trans. 6 n hcl at high t = degrade protein. Cyanogen bromide: cleave amide bonds on c terminal of methionyl residues. Edman degradation: peptide could have amino acid sequence determined by stepwise release. R1 ( n terminus)- before aa or r2 (c terminus)- after aa. Trypsin cleaves r1 site of lys and arg. Egak oligopeptide = glu - gly - ala - lys. As ph increases overall peptide charge = more negative. Pka of side chain is influenced by environment. Use beta carbon on glutamate to make bond. Regulates cell redox state and prevents damage by ros. Dimerize by forming disulfide bond b/w two cys residues. All occurs when cell is under oxidative stress.
