Enzyme
Enzymes are proteinaceous substances which are capable of catalyzing chemical reaction of biological
origin without undergoing any change in its structure. Enzyme are called biocatalyst because of is
proteinaceous nature & since protein is produced by biological system. It will be denatured at high
temp. Speeds up reactions without being changed or used up.They are reusable. Action of enzymes is
specific. Work best at a certain range of pH and temperature.
Prosthetic group
A large number of enzymes require an additional non protein component called prosthetic group for
their efficient activity. Prosthetic groups are loosely attached to the enzyme and may be divided into
two groups – (a) Metal activators; (b) Cofactors or coenzymes. Coenzymes are organic prosthetic group,
which are generally vitamins. In holoenzymes, the apoenzyme (protein part) determines the specificity
while coenzyme (prosthetic group) determines the catalytic functional activity of enzyme
Cofactors
Cofactors are either one or more inorganic ions acts as prosthetic group in enzyme, such as Fe2+, Mg2+
, Mn2+ or Zn2+
Strongly attached with protein part of enzyme (apoenzynme)
Not act as carriers of specific functional groups such as methyl groups and acyl groups
Following are the cofactors with their corresponding enzyme.
Coenzyme
is a complex organic or metalo-organic molecule acts as prosthetic group in enzyme, such as FAD, NAD,
Biocytin, Lipoate, Pyridoxal phosphate etc, loosely or transiently attached with protein part of enzyme.
It also act as carriers of specific functional groups such as methyl groups and acyl groups
Differentiate between Cofactor and Coenzyme with example
Cofactor
Cofactors are either one or more inorganic ions acts
as prosthetic group in enzyme, such as Fe2+, Mg2+ ,
Mn2+ or Zn2+
Strongly attached with protein part of enzyme
(apoenzynme )
Not also act as carriers of specific functional groups
such as methyl groups and acyl groups
Coenzyme
is a complex organic or metalo-organic molecule acts as
prosthetic group in enzyme, such as FAD, NAD, Biocytin,
Lipoate, Pyridoxal phosphate etc
Loosely or transiently attached with protein part of enzyme.
also act as carriers of specific functional groups such as
methyl groups and acyl groups
Active sites
The active site is a specialized region of the protein where the enzyme interacts with the substrate.
The active site of an enzyme is generally a pocket or cleft that is specialized to recognize specific
substrates and catalyze chemical transformations.
The interactions between the active site and the substrate occur via the same forces that stabilize
protein structure:
hydrophobic interactions,
electrostatic interactions (charge–charge),
hydrogen bonding, and
van der Waals interactions.
It also contains the groups that directly participate in the making and breaking of bonds. These groups
are called the catalytic groups.
Catalytic groups facilitate the chemistry and provide specific interactions that stabilize the formation of
the transition state for the chemical reaction.
Therefore the interaction of the enzyme and substrate at the active site promotes the formation of the
transition state.
The active site is the region of the enzyme that most directly lowers the activation energy of the
reaction, which results in the rate enhancement characteristic of enzyme action.
Two models have been proposed to explain how an enzyme binds its substrate. In the lock-and-key
model proposed by Emil Fischer in 1894, the shape of the substrate and the active site of the enzyme
are thought to fit together like a key into its lock (Fig. 1a). The two shapes are considered as rigid and
fixed, and perfectly complement each other when brought together in the right alignment. In the
induced-fit model proposed in 1958 by Daniel E. Koshland, Jr., the binding of substrate induces a
conformational change in the active site of the enzyme (Fig. 1b). In addition, the enzyme may distort the
substrate, forcing it into a conformation similar to that of the transition state. For example, the binding
of glucose to hexokinase induces a conformational change in the structure of the enzyme such that the
active site assumes a shape that is complementary to the substrate (glucose) only after it has bound to
the enzyme. Different enzymes show features of both models, with some complementarities and some
conformational change.