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Lecture 2

BIOL 200 Lecture Notes - Lecture 2: Thiamine, Victor Henri, Pyruvate Carboxylase


Department
Biology (Sci)
Course Code
BIOL 200
Professor
Mathieu Roy
Lecture
2

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Some enzymes do not need any additional components to show full activity. However, others
require non-protein molecules called cofactors to be bound for activity. Cofactors can be
either inorganic (e.g., metal ions and iron-sulfur clusters) or organic
compounds (e.g., flavin and heme). Organic cofactors can be either prosthetic groups, which are
tightly bound to an enzyme, or coenzymes, which are released from the enzyme's active site
during the reaction. Coenzymes includeNADH, NADPH and adenosine triphosphate. These
molecules transfer chemical groups between enzymes. An example of an enzyme that contains a
cofactor is carbonic anhydrase, and is shown in the ribbon diagram above with a zinc cofactor
bound as part of its active site. These tightly bound molecules are usually found in the active site
and are involved in catalysis. For example, flavin and heme cofactors are often involved
in redox reactions.Enzymes that require a cofactor but do not have one bound are
called apoenzymes or apoproteins. An apoenzyme together with its cofactor(s) is called
a holoenzyme (this is the active form). Most cofactors are not covalently attached to an enzyme,
but are very tightly bound. However, organic prosthetic groups can be covalently bound
(e.g., biotin in the enzyme pyruvate carboxylase). The term "holoenzyme" can also be applied to
enzymes that contain multiple protein subunits, such as the DNA polymerases; here the
holoenzyme is the complete complex containing all the subunits needed for activity.
Coenzymes are small organic molecules that can be loosely or tightly bound to an enzyme.
Tightly bound coenzymes can be called prosthetic groups. Coenzymes transport chemical groups
from one enzyme to another. Some of these chemicals such asriboflavin, thiamine and folic
acid are vitamins (compounds that cannot be synthesized by the body and must be acquired from
the diet). The chemical groups carried include the hydride ion (H-) carried by NAD or NADP+,
the phosphate group carried byadenosine triphosphate, the acetyl group carried by coenzyme A,
formyl, methenyl or methyl groups carried by folic acid and the methyl group carried by S-
adenosylmethionine.Since coenzymes are chemically changed as a consequence of enzyme
action, it is useful to consider coenzymes to be a special class of substrates, or second substrates,
which are common to many different enzymes. For example, about 700 enzymes are known to
use the coenzyme NADH. Coenzymes are usually continuously regenerated and their
concentrations maintained at a steady level inside the cell: for example, NADPH is regenerated
through the pentose phosphate pathway and S-adenosylmethionine by methionine
adenosyltransferase. This continuous regeneration means that even small amounts of coenzymes
are used very intensively. For example, the human body turns over its own weight in ATP each
day.
As all catalysts, enzymes do not alter the position of the chemical equilibrium of the reaction.
Usually, in the presence of an enzyme, the reaction runs in the same direction as it would without
the enzyme, just more quickly. However, in the absence of the enzyme, other possible
uncatalyzed, "spontaneous" reactions might lead to different products, because in those
conditions this different product is formed faster.Furthermore, enzymes can couple two or more
reactions, so that a thermodynamically favorable reaction can be used to "drive" a
thermodynamically unfavorable one. For example, the hydrolysis of ATP is often used to drive
other chemical reactions. Enzymes catalyze the forward and backward reactions equally. They
do not alter the equilibrium itself, but only the speed at which it is reached. For
example, carbonic anhydrase catalyzes its reaction in either direction depending on the
concentration of its reactants.Enzyme kinetics is the investigation of how enzymes bind
substrates and turn them into products. The rate data used in kinetic analyses are commonly
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