Co-Enzymes
المؤلف:
D.M. Vasudevan, Sreekumari S., Kannan Vaidyanathan
المصدر:
Textbook of Biochemistry For Medical Students
الجزء والصفحة:
10th E ,P 42-43
2025-08-05
350
i. Enzymes may be simple proteins, or complex enzymes, containing a non-protein part, called the prosthetic group. The prosthetic group is called the co-enzyme. It is heat stable. Salient features of co-enzymes are shown in Box1.
Box1. Salient Features of Co-enzymes
ii. The protein part of the enzyme is then named the apo-enzyme. It is heat labile.
iii. These two portions combined together is called the holo-enzyme.
v. Co-enzymes may be divided into two groups
v-a. Those taking part in reactions catalyzed by oxidoreductases by donating or accepting hydrogen atoms or electrons.
v-b. Those co-enzymes taking part in reactions transferring groups other than hydrogen.
First Group of Co-enzymes
In the first group, the change occurring in the substrate is counter-balanced by the co-enzymes. Therefore, such co-enzymes may be considered as co-substrates or secondary substrates. In the example shown in Fig. 1, the substrate lactate is oxidized, and simultaneously the co-enzyme (co substrate) is reduced. If the reaction is reversed, the opposite effect will take place.
Fig1. One co-enzyme molecule can work with different enzymes
Other such examples are NADP–NADPH; FAD FADH2 and FMN–FMNH2.
Nicotinamide Adenine Dinucleotide (NAD+)
i. This is a co-enzyme synthesized from Nicotinamide, a member of vitamin B complex. The structure of NAD+ is Nicotinamide-Ribose-P-P-Ribose-Adenine. Warburg (Nobel prize, 1931) elucidated the structure of NAD+
ii. The reversible reaction of lactate to pyruvate is catalyzed by the enzyme lactate dehydrogenase, but the actual transfer of hydrogen is taking place on the co-enzyme, NAD+ (Fig.2A).
iii. In this case, two hydrogen atoms are removed from lactate, out of which one hydrogen and two electrons are accepted by the NAD+ to form NADH, and the remaining H+ is released into the surrounding medium. The hydrogen is accepted by the nicotinamide group as shown in Figure2B
Fig2. A. Reaction of lactate dehydrogenase, B. NAD+ accepts hydrogen
Second Group of Co-enzymes
These co-enzymes take part in reactions transferring groups other than hydrogen. A particular group or radical is transferred from the substrate to another substrate. Most of them belong to vitamin B complex group. A few such examples are given in Table1.
Table1. Examples of co-enzymes
Adenosine Triphosphate (ATP)
i. ATP is considered to be the energy currency in the body. Fiske and Subba Row first isolated ATP in 1926 and Lohmann in 1929 showed the importance of ATP in muscle contraction.
ii. In the ATP molecule, the second and third phosphate bonds are ‘high energy' bonds (as shown with squiggle bonds in Fig.3).
Fig3. Structure of ATP
iii. During the oxidation of food stuffs, energy is released, a part of which is stored as chemical energy in the form of ATP.
iv. The endergonic reactions are carried out with the help of energy released from hydrolysis of ATP.
Metallo-enzymes
i. These are enzymes which require certain metal ions for their activity. Some examples are given in Table 2.
Table2. Metallo-enzymes
ii. In certain cases, e.g. copper in Tyrosinase, the metal is tightly bound with the enzyme.
iii. In other cases, even without the metal ion, enzyme may be active; but when the metal ion is added, the activity is enhanced. They are called ion-activated enzymes, e.g. calcium ions will activate pancreatic lipase.
Co-factors
The term co-factor is used as a collective term to include co-enzymes and metal ions. Co-enzyme is an organic co-factor.
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