Download The Michaelis-Menten equation

2 ENZYME
They are classified into six classes:
1-Oxidoreductases
Oxidoreductases are important in the body that these reactions are
responsible for the production of heat and energy. Oxidoreductase (Alcohol
dehydrogenase),oxidize of alcohols to an aldehyde by removing two electrons
as 2H+ from alcohol to yield an aldehyde.
Ethanol + NAD⁺→Alcohol dehydrogenase→ Acetaldehyde + NADH + H⁺
Lactate + NAD⁺ → Lactate dehydrogenase →Pyruvate + NADH + H⁺
2-Transferases
Transfer functional groups between donor and acceptor molecules. Kinases
are specialized transferases that regulate metabolism by transferring
phosphate from ATP to other molecules.
Glucose + ATP → Hexokinase → Glucose-6-phosphate + ADP
Fructose + ATP → Fructokinase → Fructose-1-phosphate + ADP
3-Hydrolases
Add water across a bond, hydrolyzing it.
Lactose + H2O → Lactase → Glucose + Galactose
Maltose + H2O → Maltase → Glucose + Glucose
4-Lyases
Add water, ammonia or carbon dioxide across double bonds, or remove
these elements to produce double bonds.
Fructose-1,6-bisphosphate →Aldolase A →Glyceraldehyde-3-phosphate +
Dihydroxyacetone phosphate
5-Isomerases
Catalyze racemization of optical isomers. Carry out many kinds of
isomerization: L to D isomerization, mutase reactions (interconversion of
chemical groups) and others.
Glucose-6-phosphate → Isomerase → Fructose-6-phosphate
Glucose → Epimerase → Galactose
6-Ligases
Catalyze formation of bonds between Carbon and Oxygen, Nitrogen and
Sulphur. This reactions in which two chemical groups are joined (or ligated)
with the use of energy from ATP.
The Michaelis-Menten equation
What is Michaelis-Menten equation?
Definition: The Michaelis-Menten equation describe how reaction initial
velocity Vₒ varies with substrate concentration[S],by the following equation:
K₁
E+S
K₃
[ES] complex
K₂
E+P
K₄
E is an enzyme while S is a substrate & P is a product. Where K₁,K₃ are forward
reaction rate constant. Where K₂,K₄ are reverse reaction rate constant [
V
[S]
Vₒ=Initial Velocity, Vmax =Maximum Velocity, [S]=Substrate concentration, and
Km = Michaelis-Menten Constant is to measure of how efficiently an enzyme
converts a substrate into product..
Fig (1):- Plot of reaction velocity on a function of [S] to the
enzyme by Michalis – Menten equation.
If Km numerically small (low) reflects a high affinity of the Enzyme to bind
substrate by diffusion of substrate into the active site , while if Km
numerically high reflects a low affinity of the Enzyme to substrate
concentration, Fig (1), indicate an increased rate of unbinding this in fact
decrease the reaction rate.
The Effect Of Activators and Inhibitors on Enzyme activity
INHIBITORS:Effects of Inhibitors on Enzyme Activity:
Enzyme inhibitors are substances which alter the catalytic action of the
enzyme and consequently slow down, or in some cases, stop catalysis.
Inhibitors may act combining directly with the enzyme and so effectively
remove it from the substrate (like Drugs: Heat, pH changes, strong acids,
alcohol & alkaloidal reagents cause protein denaturation, (Captopril).
e.g. The optimum temperature for most human enzymes start
between 35⁰C and 40⁰C. Human enzymes start to denature above 40⁰C
and stop its catalytic activity.
There are three types of Inhibitors :
1-Competative Inhibitor: Substances [I] that compete with the substrate[S]
for the active site of enzyme molecule and form new enzyme- substrate
complex [ES].
2-Reversbile Inhibitor: Decrease enzyme activity and full activity return
when inhibitor [I] is removed.
3-Irreversbile Inhibitor: Those inhibitor [I] binds tightly to the enzyme, and
inactivate E or destroy a functional group on the enzyme molecule, that is
necessary for its catalytic activity (enzyme inactivation), as in fig-2 below:ACTIVATORS :COFACTORS:
A cofactor is a non-protein chemical compound or metallic ion (Ca⁺², Fe⁺²,
Mg ⁺²,Mn ⁺², Zn ⁺², and K⁺), that must bind to particular enzymes before a
reaction occurs, Cofactors can be sub classified as either inorganic ions
binding cofactors or complex organic molecules binding cofactors called
coenzymes. Cofactors can be considered "helper molecules" that assist in
biochemical transformations, fig-3.
Fig-2:Types of enzyme inhibitors.
Some enzymes containing or requiring metal ions as
─────────────────────────
Fe⁺² or Fe⁺ᵌ :
Peroxidase
Zn⁺² : Alcohol dehydrogenase.
─────────────────────────
Some coenzymes in group transferring reactions:Coenzyme
Entity transferred
Flavin mononucleotide
Hydrogen atom (electron(
Figure 3:-metal ions as cofactores.
ACTVIATORS:ISOENZYMES:
Some of the enzymes are present in more than one form having the same
molecular weight and differ in conformational structures called isoenzymes,
e.g. Trypsinogen isoenzymes are present in three conformational structures :1- cationic Trypsinogen
2- anionic Trypsinogen
3- mesotrypsinogen
These conformational structures of isoenzymes are capable of digesting the
cell and causing significant damage. But there are mechanisms to prevent
these enzymes from potentially digesting the pancreas including:
storage and packing in acidic media to inhibit enzyme activity synthesis and
storage as inactive precursor forms.
some of the enzymes that are stored in the pancreas before secretion as
inactive precursor forms, then activated when they enter the duodenum.
Activation of these enzymes takes place in the surface of the duodenal lumen,
microvilli where Enterokinase, activates Trypsinogen by removing (by
hydrolysis) an N-terminal hexa peptide fragment of the molecule (Val–Asp–
Asp–Asp–Asp–Lys). The active form, Trypsin, then catalyzes the activation of
the other inactive proenzymes. Of note, many key digestive enzymes, such as
α-amylase and lipase, are present in the pancreas in their active forms.
Presumably, these enzymes would not cause pancreatic cellular damage if
released into the pancreatic cell/tissue because there is no starch, glycogen or
triglyceride substrate for these enzymes in pancreatic tissue.
Coenzyme:
Coenzymes are organic cofactors. They are Coenzymes serve as a second
substrates for enzymatic reactions, such as nucleotide phosphates and
vitamins. When bound tightly to the enzyme, coenzymes are called
prosthetic
groups. For example, NAD as a cofactor may be reduced to nicotinamide
adenine dinucleotide phosphate (NADH) in a reaction in which the primary
substrate is oxidized (the equation below). Increasing coenzyme
concentration will increase the velocity of an enzymatic reaction.
Holoenzyme:
When bound tightly to the enzyme, the coenzyme is called a prosthetic
group. The enzyme portion (apoenzyme), with its respective coenzyme,
forms a complete and active system, a holoenzyme.
Zymogen:
Some enzymes, mostly digestive enzymes, are originally secreted from the
organ of production in a structurally inactive form, called a proenzyme or
zymogen. Other enzymes later alter the structure of the zymogen to make
active sites available by hydrolyzing specific amino acid residues. This
mechanism prevents digestive enzymes from digesting their place of
synthesis. Trypsinogen, is a precursor of trypsin, its a storage of an inactive
form of trypsin so that it may be kept in the pancreas and released in
significant amount when required for protein digestion. Trypsin is formed
in the small intestine when its proenzyme Enterokinase produced by
pancreas.
This figure indicate the activation of
Inactive Trypsinogen into Trypsin
in small intestine by Enterokinase..