1. Enzyme Classification:
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
محاضرة الانزيمات 3

2. 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.

3. 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:
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 [
The Michaelis-Menten equation:
Vmax . [S]
Vₒ= ―――――
Km + [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..
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.
K₁ K₃
E + S [ES] complex E + P
K₂ K₄
[S] V
Fig (1):- Plot of reaction velocity on a function of [S] to the enzyme by Michalis – Menten equation.

4. The Effect Of Activators and Inhibitors on Enzyme activity
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.

5. 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.

6. 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.

7. 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..

8. METABOLIC DIGESTIVE ENZYMES
Amylase catalyses the breakdown of starch into sugars in the small intestine, proteases catalyse the breakdown of proteins into amino acids in the stomach and small intestine. lipases catalyse the breakdown of fats and oils into fatty acids and glycerol, in the small intestine, table -1.
The digestive enzymes begin in mouth, then by stomach acid, the stomach produces hydrochloric acid. this helps to begin digestion, and it kills many harmful microorganisms that might have been swallowed along with the food. The enzymes in the stomach work best in acidic conditions, in other words, at a low pH. Bile after the stomach, a substance called bile neutralises produced by the liver and stored in the gall bladder, for neutralises the acid to provide the alkaline conditions needed in the small intestine. the acidic food travels to the intestine. The enzymes in the intestine work best in alkaline conditions, to continue digestion to the Large intestine - colon where water is reabsorbed Then to the Large intestine – rectum, where faeces are stored finally where faeces leave the alimentary canal Figure -4 .
Fig (4):-Bile and Enzyme production
in the liver and pancreas.
Hydrolysis Enzymes
Enzymes
Enzymes activity
Location
CARBOHYDRASES
carb.→ simple sugars
Ptyalin or salivary amylase
hydrolysis of starch to dextrin and maltose.
Pancreatic juice
LIPASE AND ESTERASE:
Fat or esters → fatty acid & alcohol
Gastric Lipase
hydrolysis of fats to fatty acids and glycerol
Gastric Juice
PROTEASES: proteins → derived protein & amino acids.
Enterokinase.
Converts Trypsinogen into
trypsin in the small intestine.
released by the pancreas into duodenum → trypsin
Table -1:Some Digestive Enzymes and Their Enzymes Activities.

9. Renin-Angiotensin system (RAS)
KIDNEY ENZYMES
Renin-Angiotensin system (RAS)
If an individuals blood pressure drops as in case of hemorrhaging the kidneys secret the enzyme Renin (some times considered as a hormone) into the blood stream.
Angiotensinogen → by Renin enzyme → Angiotensin I
Angiotensin I → Angiotensin - converting enzyme → Angiotensin II
Angiotensin I = Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu | Val-Ile-.
Angiotensin II = Asp-Arg-Val-Tyr-Ile-His-Pro-Phe | His-Leu.
The function of Angiotensin-converting enzyme:
1-Angiotensin-converting enzyme, ACE is a zinc metalloenzyme. The zinc ion Zn⁺² is essential to its activity, since it directly participates in the catalysis of the peptide hydrolysis. Therefore, ACE can be inhibited by metal-chelating agents. ACE inhibitors are widely used as pharmaceutical drugs for treatment of cardiovascular diseases.
2-ACE is a central component in the plasma which requires chloride ion for its activation, and in controlling blood pressure, it regulates the volume of fluids in the body. ACE converts the hormone angiotensin I to the active vasoconstrictor angiotensin II, so ACE indirectly increases blood pressure by causing blood vessels to constrict.
For this reason, drugs known as ACE inhibitors (Captopril) are used to lower blood pressure.