Subject: Enzyme Kinetics - part 1
Date: 2000-11-01 14:12:21 GMT
There are very tiny bits of background knowledge that will make this paper
more readable. Some basic chemistry and a trivial knowledge of
differential calculus is useful in appreciating the detailed argument, but
I have tried to convey the meaning in word as well as equation and so a
skipping over any confusing equations should not impair an understanding of
the larger issues. I have tried to strike a balance between the assumption
that the audience understands the basics and that it does not. I am sure
that this balance is imperfect.
Enzymes -- rate of reaction as related to brewing
Enzymes are proteins which act as catalysts for certain chemical reactions.
Enzymes are an intimate part of virtually every biological pathway. When
one hears the term gene, or genetic property, one should think 'enzyme',
since genes are principally DNA encoding for the proteins that represent
enzymes. Plant and animal metabolic processes involve systems of thousands
of distinct enzymes to accomplish the various functions involved.
Plants store their energy reserves as carbohydrates, and many of the more
advanced plants create complex carbohydrates such as starch as part of a
long term storage strategy or a reproductive success strategy. The feed
grains fall in this last category. These starch making plants also have
enzymes which help reduce those carbohydrates to simple sugars, especially
glucose, which fuels the primary energy mechanism available to higher plants
The catalysis by enzymes of chemical reactions is a simple yet remarkable
thing. The chemical reactions involved must be thermodynamically favorable,
or exothermic. Catalysis does not provide energy to the system, but it
lowers the threshold energy level needed for a reaction to take place. By
analogy, catalysis is like lowering the level of a dam and permitting the
water to flow downhill at an increased rate. Catalysis does not cause water
to flow uphill, tho' be aware that the reverse reaction rate, or
can occasionally be significant. It cannot be said that catalysis is
necessary for the reaction as some water flows over the dam anyway. In
biological systems tho' catalysis often increases the rate of the reaction
by 3 to 6 orders of magnitude and sometimes much more. In our primary
example system using amylases to produce sugars from starch it is easy to
understand that the natural rate of starch degradation into sugar is
thousands of times lower in the absence of enzymes.
Another aspect of these systems of enzymes is that they are often part of a
regulatory system that is involved in keeping some chemical or product at an
appropriate level in a living organism. Not only must the enzymes be
available for driving the reaction forward, but it must also be possible to
stop or inhibit the reaction from going too far. A barley seed needs
glucose to fuel its growth, but the entire endosperm is consumed over a few
weeks, not an hour. There are several ways in which this is effected, but
product inhibition, in which an enzymes ability to catalyze is reduced in
the presence of either its immediate product or more often the product
several steps beyond the current one is very common.
The enzyme proteins are long chained linkages of the 20 basic types of amino
acids, and the resulting molecules may consist of up to several thousand
amino acids. The protein molecule is not necessarily a single strand, but
may contain doubled or tripled sections. The molecule is unlikely to be a
straight chain, but instead spirals and folds back on itself repeatedly,
often looking more like a jumble of ribbon. There are several sorts of
relatively weak bonds which may occur between crossing strands. These can
act to fix the ribbon jumble into a fairly fixed shape. The individual
amino acids do not carry uniform electrical potential, and so various parts
of the molecule attract and repel each other based on this local potential.
The pH of the medium and the availability of free ions (e.g. salt solutions)
impact the effect of these charges and so the shape and solubility of the
proteins. Some enzyme co-factors, agents which permit or improve the
catalytic effect, act to 'improve' the enzymes molecular shape so that it
catalyses its reaction better. One example is that many plant
alpha-amylases require calcium as a co-factor. If no calcium is present
the alpha-amylase converts into an unusable shape. (denatures) after which
it no longer catalyses its reaction and so ceases to be an enzyme. Most
cofactors impact the rate of the reaction, and are not absolutely required,
as is the case for alpha-amylase and calcium.
When we brewers speak of enzyme catalyzed reactions in the mash tun the
discussion often becomes obscure due to a lack of careful terminology,
especially about rates of reaction, so incorrect conclusions are often
derived. The concepts behind the reactions and their rates are so basic
that a Socratic dialog could lead even the least scientific brewer among us
to the correct conclusions. Unfortunately a history of catch phrases and
statements drawn upon out of context make erroneous conclusions more
common than correct ones in this area.
Some basic terminology will be introduced as needed.
- Substrate: The term 'substrate' refers to the source material that is
acted upon. The substrate of beta-amylase is linear 1-4 linked glucose
units of length 3 or greater and water. The substrate of beta-glucanase
are the 1-3 and 1-4 linkages in beta-glucans + H2O.
- Concentration, or number of molecules per unit volume, of a molecule 'M'
will be written as [M]. Often units of moles per liter are used by
chemists, where a mole represents 6.022*10^23 molecules.
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