Download ENZYME STRUCTURE AND FUNCTION

ENZYME STRUCTURE AND FUNCTION
Learning Objectives:
By the end of the lecture, the student should be able to :
•
Define enzymes and related terms ( holoenzyme, apoenzyme, prosthetic
group, isoenzymes, active site, allosteric site, specificity, enzyme activity,
regulation, location).
•
Describe the structure of enzymes.
•
Define energy of activation and energy barrier.
•
Justify that enzymes provide an alternate pathway with lower energy
barrier .
•
Describe Km.
•
Justify that enzyme affinity and Km are reciprocally related.
•
Discuss the clinical significance of Km (hexokinase & glucokinase ).
•
Discuss the general clinical significance of enzymes.
INTRODUCTION
 Enzymes play an important role in Metabolism, Diagnosis, and Therapeutics. All
biochemical reactions are enzyme catalyzed in the living organism.
 Level of enzyme in blood are of diagnostic importance e.g. it is a good indicator
in disease such as myocardial infarction.
 Enzyme can be used therapeutically such as streptokinase in myocardial
infarction and digestive enzymes.
DEFINITION OF ENZYME
 ENZYME:- They are biological catalyst unchanged at the end of reaction, require
only small amount do not change the equilibrium but equilibrium is reached faster
in the presence of enzyme, they are specific.
 Almost all the enzyme are protein catalyst, they catalyzed small number of
reaction, sometime one specific reaction.
COENZYME
 certain enzymes require a specific thermostable, low molecular weight non
protein organic substance called coenzyme.
Space-filling model of the coenzyme NADH
ISOENZYMES
 Isoenzyme or isozymes are physically distinct forms of the same enzyme but
catalyze the same chemical reaction or reactions but differs from each other
structurally, electrophoretically and immunologically.
ACTIVE SITES
Enzyme molecules contain a special pocket or cleft called the active sites. The active
site of an enzyme is usually found in a cleft or pocket that is lined by amino
acid residues that participate in recognition
of the substrate. The active site binds the
substrate forming the enzyme substrate
(ES) complex. ES complex is converted to
enzyme product (EP) complex.
HOLOENZYME
Holoenzyme is an active enzyme with its non protein component, whereas the enzyme
without its non protein moiety is termed as apoenzyme and it is inactive. If its non
protein moiety is metal e.g. zinc or iron it is called cofactor. If the coenzyme is
permanently associated with enzyme and returns to its original form it is called
prosthetic group e.g. FAD.
Allosteric sites
Allosteric sites are sites on the enzyme that bind to molecules in the cellular
environment. The sites form weak, noncovalent bonds with these molecules, causing a
change in the conformation of the enzyme. This change in conformation translates to
the active site, which then affects the reaction rate of the enzyme. Allosteric interactions
can both inhibit and activate enzymes and are a common way that enzymes are
controlled in the body.
SPECIFICITY
Enzymes are usually very specific as to which reactions they catalyze and
the substrates that are involved in these reactions. Because enzymes are extremely
selective for their substrate the sets of enzymes made in cell determines which
metabolic pathway occurs in the cell.
Structures and mechanisms
Enzymes are generally globular proteins and range from just 62 amino acid residues in
size, for the monomer of 4-oxalocrotonate tautomerase However, although structure
does determine function, predicting a novel enzyme's activity just from its structure is a
very difficult problem that has not yet been solved.
Most enzymes are much larger than the substrates they act on, and only a small portion
of the enzyme (around 2–4 amino acids) is directly involved in catalysis. The region that
contains these catalytic residues, binds the substrate, and then carries out the reaction
is known as the active site. This binding can serve to increase or decrease the
enzyme's activity, providing a means for feedback regulation.
Like all proteins, enzymes are long, linear chains of amino acids that fold to produce
a three-dimensional product.
Each unique amino acid sequence produces a specific structure, which has unique
properties.
Individual protein chains may sometimes group together to form a protein complex.
Most enzymes can be denatured—that is, unfolded and inactivated—by heating or
chemical denaturants, which disrupt the three-dimensional structure of the protein.
Depending on the enzyme, denaturation may be reversible or irreversible.
REGULATION
Enzyme activity can be regulated that is enzyme can be activated or inhibited so that
the rate of product formation responds to the needs of the cell.
LOCATION WITHIN THE CELL.
Many enzymes are localized in specific organelles within the cell, such
compartmentalization serves to isolate the reaction substrate or product from competing
reaction.
ENERGY OF ACTIVATION AND ENERGY BARRIER.
All chemical reaction have an chemical reactions have an energy barrier separating the
reactant and the product. This barrier called the free energy of activation is the energy
difference between that of the reactants and high energy intermediates that occurs
during the formation of product.
AULTERNATE REACTION PATHWAY
Enzyme allowed to proceed rapidly under condition prevailing in the cell by providing an
alternate reaction with lower free energy of activation the enzyme does not change the
free energy of activation of the reactant or product and therefore does not change
equilibrium of the reaction. It accelerate the rate with which equilibrium.
Km
The Michaelis constant (Km) is a means of characterizing an enzyme's affinity for a
substrate. The Km in an enzymatic reaction is the substrate concentration at which the
reaction
rate
is
half
its
maximum
speed.
Thus, a low Km value means that the enzyme has a high affinity for the substrate (as a
"little" substrate is enough to run the reaction at half its max speed).
This is only true for reactions where substrate is limiting and the enzyme is NOT
allosteric.
CLINICALSIGNIFICANCE
Lactose Dehydrogenase (LDH) isoenzymes, catalyzes reversible oxidation of lactate to
pyruvate, in serum as many as five physically distinct Isoenzyme of this enzyme exist
they are;
LDH-1, LDH-2, LDH-3, LDH-4, LDH- 5.
Creatinine PhosphoKinase (CPK) has three isoenzymes.
 CPK – 1 BB for brain.
 CPK- 2 MB for myocardium.
 CPK- 3
MM for skeletal muscles.
isoenzymes of alkaline phosphatase (ALP)
 Hepatic ALP.
 Bone ALP.
 Intestinal ALP.
Placental ALP.
CLINICAL SIGNIFICANCE OF Km HEXOKINASE OR GLUCOKINASE
A hexokinase is an enzyme that phosphorylates a six-carbon sugar, a hexose, to a
hexose phosphate. In most tissues and organisms, glucose is the most
important substrate of hexokinases, and glucose-6-phosphate the most important
product.
Glucokinase is an enzyme that facilitates phosphorylation of glucose to glucose-6phosphate. Glucokinase occurs in cells in the liver, pancreas, gut, and brain of humans.
In each of these organs it plays an important role in the regulation of
carbohydrate metabolism by acting as a glucose sensor, triggering shifts in metabolism
or cell function in response to rising or falling levels of glucose, such as occur after a
meal. All of the hexokinases can mediate phosphorylation of glucose to glucose-6phosphate (G6P), which is the first step of both glycogen synthesis and glycolysis.
Glucokinase has a lower affinity for glucose than the other hexokinases do, and its
activity is localized to a few cell types, leaving the other three hexokinases as more
important preparers of glucose for glycolysis and glycogen synthesis for most tissues
and organs.
REFERENCES: Lippincott – Chapter 5.