There are a few theories explaining the mechanism of action of enzymes. Perhaps each of them tries to view the fact from different perspectives to explain a particular aspect of the action.

 1. Lowering of Activation Energy

 i. Enzymes lower the energy of activation.

ii. Activation energy is defined as the energy required to convert all molecules of a reacting substance from the ground state to the transition state.

 iii. Substrates are remaining in an energy trough, and are to be placed at a higher energy level, whereupon spontaneous degradation can occur. Suppose, we want to make a fire; even if we keep a flame, the wood will not burn initially; we have to add kerosene or paper for initial burning. Similarly, the activation energy is to be initially supplied.

 iv. During enzyme substrate binding, weak interactions between enzyme and substrate are optimized. This weak binding interaction between enzyme and substrate provides the major driving force for the enzymatic catalysis.

v. Enzymes reduce the magnitude of this activation energy. This can be compared to making a tunnel in a mountain, so that the barrier could be lowered (Fig. 1). For example, activation energy for acid hydrolysis of sucrose is 26,000 cal/mol, while the activation energy is only 9,000 cal/mol when hydrolyzed by sucrase.

Fig1. Lowering of activation energy by enzymes

2. Acid Base Catalysis

Protonated form of histidine is an example of a general acid and its conjugate base, the general base (Fig.2). The action of ribonuclease is an example of acid-base catalysis. Histidine residues 12 and 119 at the active site of ribonuclease function as acid and base in catalysis. Histidine 12 acts as an acid and donates a proton to form the basic form. Then the 2’-3’ cyclic phosphate is formed. Histidine 112 accepts a proton from the cyclic phosphate and product is released.

Fig2. Acid base catalysis with the help of histidine

In the enzymes of aspartyl protease family, catalysis involves two aspartyl residues, which act as acid-base catalysts. Catalysis by pepsin, cathepsin and protease of HIV (human immunodeficiency virus) belong to this group of enzymes.

3-A. Substrate Strain

Binding of substrate to a preformed site on the enzyme can induce strain in the substrate. The energy level of the substrate is raised. A combination of substrate strain and acid base catalysis is seen in the action of lysozyme.

The lysozyme substrate has a repeating hexasaccharide unit. Binding of the substrate to the enzyme generates a strained conformation in the enzyme substrate complex (D in Fig.3). In the transition state, acid catalyzed hydrolysis of the glycosidic linkage by a glutamic acid residue at the active site generates a carbonium ion on the D residue. This relieves the strain generated in the initial enzyme-substrate complex. This results in the change from transition state to products. The glycosidic bond between N-acetyl glucosamine and N acetyl muramic acid on the bacterial cell wall is thus hydrolyzed. This accounts for bactericidal action of lysozyme. Lysozyme was purified and studied by Howard Florey (Nobel prize, 1945).

Fig3. Active center of Lysozyme. A,B,C,D,E,F are carbohydrate units (substrate). D = N-acetyl muramic acid; E= N-acetyl glucosamine. Bond is broken between D and E, with the help of Glu and Asp residues in the enzyme, which are opposite to each other

3-B. Serine proteases

They are enzymes with a serine residue at the active site and most of the proteolytic enzymes belong to this group, e.g. trypsin, chymotrypsin, clotting factors (Table1).

Table1. Proteases

3-C. Action of chymotrypsin It is a combination of covalent and acid base catalysis. The peptide bond is hydrolyzed through acid catalyzed nucleophilic attack, utilising the serine 195 residue of the enzyme. The enzyme substrate complex is formed by the binding of the aromatic amino acid residue with the hydrophilic pocket on the active site on the enzyme. A covalent acyl enzyme intermediate is formed. Cleavage of the peptide bond occurs. Serine 195, Histidine 57 and Aspartate 102 are the catalytic groups.

 4. Covalent catalysis In covalent catalysis, a nucleophilic (negatively charged) or electrophilic (positively charged) group of the enzyme attacks the substrate. This results in covalent binding of the substrate to the enzyme. Similarly, co-enzymes often form covalent bonds with the substrates.

 5. Entropy effect Enzymes enhance reaction rates by decreasing entropy. When correctly positioned and bound on the enzyme surface, the substrates are strained to the transition state. This is referred to as the proximity effect. Chemical reactions need physical apposition of two reactants. The occurrence of collision between two substrate molecules is determined by statistical probability. Since substrates usually are present in low concentrations, the collision probability is less and hence the reaction velocity is low. But a complex formation between the enzyme and the two substrate molecules can improve the collision probabilities many fold, causing the rapid rate of reaction.

6. Product Substrate Orientation Theory Enzyme has appropriate three-dimensional structure to keep the substrates in a specific orientation, such that the reactive groups come in to physical apposition, leading to speedy reactions (Fig.4). It has been shown that the hydroxyl group of the 6th carbon atom of glucose and the terminal phosphate group of ATP are juxtaposed with the help of hexokinase.

Fig4. Correct alignment of amino acids in the active center of the enzyme

اخر الاخبار

اخبار العتبة العباسية المقدسة

شعبة متابعة الأداء تنظم محاضرة تربوية لمتطوعات مجمع العلقمي استعدادًا لزيارة الأربعين

شعبة الحرم الشريف تكثف استعداداتها لتقديم خدمات متنوعة لزائري الأربعين

قسم الشؤون الفكرية يفتتح محطتين ثقافيتين في ذي قار لخدمة زائري الأربعين

العتبة العباسية المقدسة تعقد ملتقى خاصًا لمنتسبيها العاملين في مراكز إرشاد التائهين خلال زيارة الأربعين

المزيد

الآخبار الصحية

علامات فارقة بين النوبة أو السكتة القلبية.. ماهي؟

الفلفل الأحمر.. سعرات حرارية منخفضة وفوائد صحية عديدة

8 أعراض إذا شعرت بها.. اذهب للطوارئ فورا !

تعرف على أكثر 8 أماكن اتساخا في المنزل

المزيد

الآخبار التكنلوجية

إنتاج الهيدروجين الأخضر في البحر.. هذه أحدث الدراسات

"ناسا" تخطط لبناء مفاعل نووي على سطح القمر !

ابتكار ومواهب وأموال.. مارك زوكربيرغ يعلن الحرب على أبل

عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

المزيد

مواضيع ذات صلة

Co-Enzymes

Classification OF Enzymes

Regulatory Covalent Modifications can be Reversible or Irreversible

Allosteric Effectors Regulate Certain Enzymes

الأهمية الحيوية الطبية للأنزيمات

انزیم ( Glutamate Pyruvate Transaminase or Alanine Transaminas) GPT or ALT

أنزيم GOT ( AST ) (Glutamate Oxaloacetate Transaminase or Aspartate Transaminase)

كيف يعمل الأنزيم

فرضية القفل والمفتاح Lock and Key hypothesis

تصنيف الانزيمات Classification of Enzymes

الانزيمات Enzymes

Initial Considerations and Primary Recovery