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pH and Buffers
Bonnie Ownley
Department of Entomology and Plant Pathology
The University of Tennessee, Knoxville
What is pH?
Measure of the concentration of hydrogen ions
(H+) in a solution
Why is pH important?
All living things on earth are water-based systems,
and are very dependent on acid-base equilibrium.
The cell environment is always buffered at pH 7.0.
The pH scale ranges from 0 (extremely acid) to
14 (extremely basic).
Acidic conditions = 0 to 6.9.
Alkaline conditions = 7.1 to 14.0
The pH scale is logarithmic, so that for each 1
unit change in pH, there is actually a ten-fold
change in acidity or alkalinity.
http://www.tuscany-diet.net/wp-content/uploads/2013/08/pH-Scale.jpg
How does this relate to proteins?
In biology, macromolecules, such as proteins, act
as acids and bases by accepting and donating
protons.
Enzymes (proteins) require a certain pH level for
activity.
Enzymes are large molecules and have acidic and basic
groups, rather than just being an acid or a base.
Hydrophobic
Hydrophilic
Basic
Acidic
http://xray.bmc.uu.se/Courses/bioinformatik2003/Intro/aminoacids.gif
On protein (enzyme) molecules, changes in pH
determine which groups are protonated (have an
attached hydrogen ion).
Enzyme
At low pH (acidic
conditions), the -NH3+
group will not be affected,
but the -COO- will pick up
a hydrogen ion.
Enzyme
In this case, we have lost the
ability to form ionic bonds
between the substrate and the
enzyme. If those bonds were
needed to attach the substrate
and activate it, then at a lower
pH, the enzyme will not work.
Enzyme
If pH is higher than 7
(alkaline conditions), the COO- group will not be
affected, but the -NH3+ group
will lose a hydrogen ion.
http://chemwiki.ucdavis.edu/Biological_Chemistry/Proteins/B._Proteins/4._The_Effect_of_Changing_Conditions_on_Enzyme_Catalysis
What is a buffer?
• Usually a weak organic acid and its salt
• Acts to protect against abrupt changes in
acidity or alkalinity
Buffer for Exercise
Phosphate buffered saline
Components:
NaCl (sodium chloride)
KCl (potassium chloride)
KH2PO4 (monopotassium phosphate)
Na2HPO4×7H2O (disodium phosphate)