Hello jat,
I assume that you already know what an enzyme is, but for the purpose
of review, and to get a better grasp on the question, I would like to
start with a definition of an enzyme:
“Enzymes act as catalysts for the chemical reactions that occur in
living organisms, allowing reactions to occur in the milliseconds
necessary to life. Chemically, enzymes are proteins. Each specific
enzyme has a unique physical structure that is essential for its
function. The shape of each specific enzyme “fits” the shape of the
reacting molecule(s) for which the enzyme serves as a catalyst.
Because of the enzyme “fit”, the reacting molecules are brought
together at the appropriate bonding sites. The enzyme, therefore,
“lowers” the activation energy of the chemical reaction.”
Source: http://scidiv.bcc.ctc.edu/rkr/Biology101/labs/pdfs/Enzyme101.pdf
We know that enzymes are proteins, and you probably know that proteins
have a primary, secondary, tertiary and sometimes quaternary
structures. If you need to brush up on this, please read through this
page before continuing:
http://web.mit.edu/esgbio/www/lm/proteins/structure/structure.html
There is a neat graph here which shows the effects of pH on the
enzymes pepsin and chymotrypsin:
http://dwb.unl.edu/Teacher/NSF/C10/C10Links/www.fordham.edu/Biochem_3521/lect10/pH.html
It does seem that enzymes tend to like a lower (more acidic) pH, “For
instance, pepsin that is needed in the stomach has an acidic optimum
and would work in lemon juice.”
Source: http://newton.dep.anl.gov/askasci/bio99/bio99993.htm
There are of course exceptions:
“pH optima also vary greatly between enzymes. It is rare to find pH
optima below 3.0 or above 9.0. Many hydrolases have optima of around
4.5-6.0. Transferases often have pH optima around 7.0. The shape of
the pH-activity curve is governed by the individual titration curves
for the amino acid residues involved in substrate binding, catalysis
and in maintaining an active conformation for the enzyme.”
http://www.fst.rdg.ac.uk/courses/fs355/frame6.htm
My research has shown that it is not so much a direct contribution
that a high or low pH has on an enzyme (which is not optimal to the
particular enzyme), but that pH levels which are at extremes have the
ability to destroy or disrupt enzyme activity (thus causing it to be
less successful). The reason that performance decreases at a
non-optimal pH level is because the enzyme folding (in the secondary,
tertiary and quaternary levels) is being broken or changed.
I found a PDF file from the School of Medicine at the Southern
Illinois University that gives two “gross effects” of pH on enzyme
activity:
“1). Extreme pH, Acid or Alkali, will partially denature many enzymes
due to mutual repulsion between like charges (+ ve at low pH and –ve
at high pH). This may unfold secondary and tertiary structure and
disrupt the active site, and so inactivate the enzyme. These effects
may or may not be irreversible.”
“2). Less extreme pH may alter the quaternary structure of a protein
by dissociating subunits, without causing major loss of second and
third structure, but this may still modify enzyme activity,
particularly cooperative effects that require subunit-subunit
interactions. Often Reversible.”
Source: http://www.siumed.edu/~phardwicke/451bpdf/EFFECTOFpHOH.pdf
There are some exceptions to this however as can be seen in this
example: “In many enzymes, the ionization state of the amino acid side
chains are important for catalytic activity. In the case of trypsin,
the His residue in the active site must be deprotonated in order for
it to act as a general base catalyst.”
http://info.bio.cmu.edu/courses/03231/LecF02/Lec26/pHEffects.pdf
In the above example, pH directly has an effect on the active site,
and therefore it is important to have the proper pH (or be in the
proper acceptable range) to get the enzyme working in the first place.
(This is not to say that the enzyme would be destroyed when the pH is
such that it does not work, although that would indeed happen at an
extreme pH). Here is another quote which may make this more clear:
“Changes in pH alter the state of ionization of charged amino acids
(e.g., Asp, Lys) that may play a crucial role in substrate binding
and/or the catalytic action itself. Without the unionized -COOH group
of Glu-35 and the ionized -COO- of Asp-52, the catalytic action of
lysozyme would cease.”
Source: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Enzymes.html
To summarize, In cases where pH is required to configure the active
site first, pH plays a role in jumpstarting the enzyme function. The
more obvious effect that pH has on enzyme activity however, is that
deviations from the optimal pH can change the enzyme causing it to
reduce the quality/speed of the process, or with an extreme pH,
completely disrupt the enzyme. This happens because the pH messes
with the protein structures and reconfigures them is such a way that
they do not work, or that they do something completely different.
I have selected some additional links which lead to some good articles
on pH effects on enzyme activity if you have a more advanced interest:
http://www.uic.edu/classes/bios/bios100/labs/enzyme.htm (see Exercise
3 for an example on how to determine optimal pH)
http://www.sbu.ac.uk/biology/enztech/ph.html (impressive article which
although fairly advanced, does a super job of carefully explaining pH
effects on Enzymes)
http://www.bio.mtu.edu/campbell/bl4010/4010lec15b-nu1c.pdf (this
article has some good visuals as well as discusses some specifics in
reference our topic)
I hope this was the type of answer you were expecting. Please let me
know if you require any clarifications and I will do my best to
further assist you.
tisme-ga
Search Strategy:
ph enzyme OR enzymes
://www.google.com/search?q=ph+enzyme+OR+enzymes
ph enzyme activity
://www.google.com/search?q=ph+enzyme+activity
protein primary secondary quaternary
://www.google.com/search?q=protein+primary+secondary+quaternary
ph enzyme activity graph OR graphs
://www.google.com/search?q=ph+enzyme+activity+graph |
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