BPS 4104 Chapter Notes - Chapter 10: Isoelectric Point, Carboxylic Acid, Zwitterion
17 May 2018
School
Department
Course
Professor
Chapter 10: Protein Isoelectric Points & Acid Resistance in H. pylori
Amino Acids & Protein Isoelectric Point
-ionization constant (Ka) measures the tendency of a chemical to give up a proton
-for example: an ionizing reaction involving a weak acid:
-R-COOH=RCOO- + H+
-then Ka is expressed as:
-
-this will be re-designated as Ka1 for the ionization constant in anaminoacid
ionization reaction involving a weak acid
-ionization constant involving a weak base will be designated as Ka2
-if we take the base-10 log of both sides of the Ka equation above, we have:
-
-pH is defined as –log[H+] and pKa1 as –logKa1
-the above equation then becomes the Henderson-Hasselbalch equation:
-
-for weak bases such as amines, the Henderson-Hasselbalch equation is:
-
-an amino acid is a weak base and a weak acid at the same time
-exists as a cation at low pH and anion at high pH
-an amino acid at an intermediate pH when it carries no net charge is called a
zwitterion:
-
-the isoelectric point of an amino acid is defined as the pH at which the amino acid
exists as a zwitterion
-occurs when [RCOO-] and [NH3+] are equal
pH-pKa1+log[RCOOH]=pKa2-pH+log[NH2]
2pH=pKa1+pKa2+log[NH2]-log[RCOOH]
2pH=pKa1+pKa2+log([AA]-[NH3+])-log([AA]-[RCOO-])
pH=
find more resources at oneclass.com
find more resources at oneclass.com
-final pH is designated isoelectric point (when [NH3+ and RCOO- are equal)
-each peptide has an H2N group at one end and a COOH group at the other
-the charge of the peptide depends mainly on the ionization of the side chain of
amino acid residues
-there are only seven amino acids that are relevant when calculating pI (in table
above)
-how to compute pI:
-count the frequencies of the seven ionisable amino acid residues
-for each amino acid, we compute the proportion of positively charged &
negatively charged residues, designated by PNH3+ and PCCO-
-
-
-we can then rearrange these equations to obtain:
-PRCOO- can be interpreted in two ways:
-proportion of residues that carry a negative charge at given pH
-probability that a residue will be in a negatively charged state at given pH
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Chapter 10: protein isoelectric points & acid resistance in h. pylori. Ionization constant (ka) measures the tendency of a chemical to give up a proton. For example: an ionizing reaction involving a weak acid: This will be re-designated as ka1 for the ionization constant in anaminoacid ionization reaction involving a weak acid. Ionization constant involving a weak base will be designated as ka2. If we take the base-10 log of both sides of the ka equation above, we have: Ph is defined as log[h+] and pka1 as logka1. The above equation then becomes the henderson-hasselbalch equation: For weak bases such as amines, the henderson-hasselbalch equation is: An amino acid is a weak base and a weak acid at the same time. Exists as a cation at low ph and anion at high ph. An amino acid at an intermediate ph when it carries no net charge is called a zwitterion:
