Download - General and systemic Biochemistry Enzyme
Tishk International University
Faculty of Science
Department of Medical Analysis
General and systemic Biochemistry
Enzyme
Ass. Prof. Dr. Rundk A. Hwaiz
Grade 2-Fall 2021-2022
Dr. Rundk A. Hwaiz
Introduction
Enzymes are biological catalyst that accelerate the rate of chemical reacton without
being consumed or produced in the reaction
Enzymes [E ], at the first step react with substrate [S] produce complex [ES],
the second step [ES] complex convert to product [P], and [E]. So the substrate
convert to product, and the enzyme remain as the same without consume or any
change in structure, cause increase rate of chemical reaction and decrease activation
energy of the reaction.
E + S ES E +P
Properties of enzymes
• Nature: organic and mostly protein in nature. It could be RNA which called
ribosomes that act on phospho diester bond in nucleotides.
• Specifity: enzymes are specific.
• Production: enzymes produces inside the living cells but can act in and out
living cells.
• Mode of action: the enzymes function is to accelerate the rate of chemical
reaction by decreasing the energy of activation.
• Feedback: enzyme activity can be increased or decreased according the body
needs.
Proteins nature
Protein could be simple (only amino acids) or complex (there is another group
rather than protein or amino acids.
Simple enzyme like lactase which only contain amino acids.
Complex enzyme contain protein part and non protein part, all together called
Holo-enzyme. In holo enzyme the protein part called apo-enzyme, and non protein
part called co-factor. Co-factor is co-enzyme (organic substance) or metal (inorganic
substance) like Mn and Mg.
Usually the organic co-enzymes are loosly attached and usually are vitamins, and
they called Metal activated enzyme.
But if the metal is tightly attached or integrated in the formation of enzyme, and is
part of enzyme, so they called Metalo enzyme.
Functions of co-factor are:
• Activation of enzyme
• Carrier
• Doner
Terminology
Old name like pepsin
Substrate + ase like lactase
Substrate + action like lactate dehydrogenase.
Mode of action
1. Energy change:
Enzymes react with substrate to give product and accelertate the reaction
by lowering energy of activation.
2. Active site or catalytic site:
• Lock and key (fischer hypothesis): the active site is rigid shape must suits the
substrate.
• Induced fit (Koshland): the active site is not rigid nor pre-shaped, but will
change the shape of activesite to be suitable to bind to substrate.
In fischer, the enzyme shows absolute specificity, it act only on one substrate.
But in koshland, it act on several of substrate so it shows less specifity.
Enzyme specifity
1. Absolute, only one sub like argenase act on arginine.
2. Dual, act on two substrate like xanthine oxidase act on xanthine and
hypoxanthine.
3. Relative, specific to certain type of bond like lipase act on ester bond.
4. Group realtive, act on the specific bond formed by specific group, like pepsin
which act on peptide bond formed between aromatic AA or trypsin act on
peptide bonds with basic AA.
Factors affecting the enzyme and reaction velocity
Temperature
pH
Enzyme concentration
Substrate concentration
Temperature
The optimum temperature of an enzyme is that temp. at which the greatest amount
of substrate is changed in unit time . For example in the case of G.I.T. enzymes,
the optimum temp. is around 40 C.
Generally the velocity of an enzyme catalyzed reaction increases as temp. rises.
pH
Each enzyme has an optimum pH at which the enzyme reacts at maximum speed or
has the greatest activity.
Generally moderate pH changes affect the ionic states of enzyme and substrate .
Most enzymes are inactivated in strong acid or alkaline solutions, for example
pepsin loses its activity rapidly above pH 8 and sucrase below pH 3 because
enzymes are proteins and they denaturate at high acid or alkaline media.
Shortly the optimum pH of most enzymes
is between pH5 and pH 9.
Enzyme concentration
Within wide limits the speed of an enzymatic reaction is proportional to the
enzyme concentration when the substrate concentration is maintained
constant, as its illustrated by the following figure :
Substrate concentration [ S ]
If the concentration of the substrate [s] is increased while all other conditions are
kept constant, the velocity of an enzyme catalysed reaction increases to maximum
value and then no further increase, since the enzyme will be saturated with substrate.
E + S ES E +PK1
K-1
K2
Km=K-1+K2/K1
To measure the affinity between enzyme and substrate, the lower the Km the
higher the affinity between enzyme and substrate.
Enzyme inhibitors or enzyme inhibition
Any substance that can diminish the velocity of an enzyme-catalyzed reaction is
called an inhibitor.
The pharmacological action of many drugs depend largely on enzyme
inhibition.
The inhibitor resembles closely in structure to substrate so it can combines
reversibly with the enzyme forming an EnI complex instead of EnS complex which
is normally formed. When both the substrate and this type of inhibitor are present,
they compete for the same binding site on the enzyme surface.
The family of drugs known as Angiotensin Converting Enzyme ( ACE ) inhibitors
are a good example of enzyme inhibitors as drugs .
ACE
Angiotensin I Angiotensin II
S ; inactive ( decapeptide)
P; potent pressure
Capotril (decapeptide) = I
Statins
Statins are group of drugs such as Atorvastatin, they inhibit production of
cholesterol, take by patients with hypercholesterolemia, this drug inhibit the enzyme
3hydroxy, 3methyl glutaryl CoA reductase (HMG CoA reductase), this enzyme
catalyze convert HMG CoA to mevalonic acid, a rate-limiting step for cholesterol
production, so cause decrease cholesterol in blood.
CLASSIFICATION OF ENZYMES
1-Oxido-reductases
2-Transferases
3-Hydrolases
4-Lyases
5-Isomerases
6-Ligases or Synthetases
1-Oxidoreductases:
These are enzymes catalyse oxido-reduction reactions between substrates.
2H2O2 2H2O + O2 Catalase
2-Transferases:
These are enzymes catalysing reactions involving transfer of group from
molecule to other molecule, such as creatine kinase, glucokinase and others.
Creatine +ATP Creatine phosphate + ADPKinase
3-Hydrolases:
These are enzymes catalysing reactions involving hydrolysis, such as maltase,
sucrase, protease.
Maltose + H2O Glucose + GlucoseMaltase
4-Lyases:
These are enzymes catalyse the removal of atoms or groups from substrates by
mechanisms other than hydrolysis leaving double bond (s), such as fumarase.
Fumarate + H2O MalateFumarase
5-Isomerases:
These are enzymes catalyzing interconversion of optical, positional and
geometrical isomers, for example Phospho-hexoseisomerase.
G—6—P. F—6—P. Phospho-hexoseisomerase
6-Ligases or Synthetases :
These are enzymes catalysing the linking together of two elements or compounds,
depending on the chemical nature of the bond which is formed, example citrate
synthase.
Oxaloacetate + Acetyl CoA Citrate
NH3 + CO2 + 2ATP Carbamoyl phosphate+2ADP +Pi
citrate synthase
Carbamoyl phosphate synthetase
Activation of enzymes: Activation of enzyme can occur via co-factor but also
as below:
ZYMOGENS: Are ( pro-enzymes ) : these are the inactive precursors of enzymes,
examples:
Zymogens Active form of enzymes
Pepsinogen activate by HCL in stomach to Pepsin
Trypsinogen activate by enterokinase to Trypsin
Prothrombin activate by factor X to Thrombin