Apoenzyme And Enzyme? 11 Facts (Read This First!)

Apoenzyme and enzyme are two most important terms used whenever talking about catalyzes of various metabolic biochemical reactions. Here we are going to discuss about apoenzyme and enzyme along with several important facts related to them.

Apoenzyme refers to a proteinaceous component ( part of a holoenzyme) which requires addition of a cofactor (such as coenzymes, metal ions, etc) to become active as a holoenzyme and catalyzes a biochemical reaction. Whereas the enzyme or simple enzyme refers to a proteinaceous component which directly binds to its substrate and catalyzes metabolic reactions.

What is the relation between apoenzyme and enzyme?

Apoenzyme and enzyme both are important components required for catalyzes of metabolic reactions within the body.

There are two types of proteinaceous components found in cellular systems which catalyze the metabolic biochemical reactions, these are- simple enzymes and holoenzymes. The holoenzyme consists of two different parts, one proteinaceous part that is the apoenzyme and another non-protein part, the cofactor(such as coenzymes, metal ions, etc). These two parts combine to be active and catalyze various biochemical reactions. Whereas the simple enzyme is a Protein component directly catalyzes metabolic reactions.

How are apoenzymes enzymes?

Apoenzymes are an important proteinaceous part of holoenzymes, and participate in various metabolic reactions. That’s why we must understand how the apoenzymes are considered as enzymes.

There are two main types of enzymes found within the body of an organism, such as- simple enzyme and holoenzyme. Holoenzymes are required in small amounts within the body and play an important role in catalyzing the biochemical reactions. The apoenzymes are the main proteinaceous part of holoenzymes and catalyzes the rate of biochemical reactions hence they are also considered as an important part of enzymes or as enzymes itself active by the addition of cofactors.

Is apoenzyme an inactive enzyme?

Apoenzyme is the proteinaceous part of holoenzymes that participates in various metabolic reactions. Let’s discuss its activities.

The apoenzyme is an inactive part of the enzyme or specifically said holoenzyme. The apoenzyme only activates when it binds with a specific cofactor or helper molecule. The cofactor can be a coenzyme or can also be a metal ion such as  iron, magnesium, manganese, cobalt, copper, zinc ions, etc. After binding with the cofactor the apoenzyme gets activated and known as holoenzyme. Then it gets involved with various metabolic reactions.

Without binding to the cofactors an apoenzyme can bind to the specific substrate due to its affinity towards the substrate, but I’m absence of cofactors it can not transform the substrate into the product.

Can all enzymes be the apoenzymes?

As apoenzymes are an important part of a holoenzyme, it is important to know if all enzymes can be apoenzymes or not.

There are two main types of enzymes found within the body of an organism, such as- simple enzyme and holoenzyme. That means all enzymes can not only be the apoenzymes. There are apoenzymes as well as simple enzymes both are present in cellular systems within the body necessary for the catalyzes of several biochemical reactions. Enzymes that need another helping molecule (cofactor) for its complete activity are the apoenzymes only.

Where are apoenzymes found?

Apoenzyme and enzyme both are biological catalyst of cellular system, effect the rate of biochemical reactions by their presence in cell.

The apoenzymes are found within a biological system. It means in the body of a living organism within its cellular system Different apoenzymes are found. They synthesize from the cells in a small amount and after addition to the cofactor, facilitates various metabolic biochemical reactions.

How apoenzymes are produced and when?

As apoenzymes are considered as the inactive part of the holoenzymes, they are produced in the same manner similar to the simple enzymes.

The enzymes are required for the metabolic purposes of the body, it catalyzes biochemical reactions within the body to digest the food intake and facilitates absorption of nutrients. When the food materials need to be digested the enzymes start secreting from various cells such as liver cells, pancreatic cells, etc. Along with other enzymes the apoenzymes are also produced during this from the body cells. 

Functions of apoenzyme

Apoenzyme and enzyme (simple enzyme) are one of the important molecules required for efficient metabolic processes. They have several functions within a living system,such as-

  • Creation of holoenzymes
  • Catalyzing biochemical reactions

Let’s have a closer look at the functions of apoenzyme and enzyme.

Creation of holoenzymes

Apoenzymes are the major proteinaceous component of the holoenzymes which remain inactive. After binding with the cofactor (such as coenzymes or metal ions) the apoenzyme gets activated and forms a holoenzyme.

Catalyzing biochemical reactions

After binding with the cofactor (such as coenzyme or metal ions like iron, magnesium, manganese, cobalt, copper, zinc ions) it forms a holoenzyme. The holoenzyme is then capable of binding with the specific substrate and converts it into a product. Thus catalyzes biochemical reaction.

Structure of apoenzymes

As apoenzymes are the inactive part of the holoenzymes, its structure is more or less similar to the structure of simple ideal enzymes.

The apoenzymes are primarily the proteinaceous part of the holoenzymes. It is composed of carbon (C), hydrogen (H2), oxygen (O2) and nitrogen (N) molecules. The apoenzymes bind with the non-protein parts (cofactors) and get activated with its catalytic properties. Then it binds with a specific substrate and converts that into the product efficiently. 

Examples of apoenzyme

There are several different types of apoenzymes found within the body of any living organism. Some most common examples of apoenzymes are- 

  • Glucose oxidase.
  • Carbonic anhydrase
  • Cytochrome oxidase
  • Pyruvate kinase
  • Pyruvate carboxylase
  • Monoamine oxidase
  • Catalase
  • Lactate dehydrogenase
  • DNA polymerase

Glucose oxidase

Glucose oxidase is one of the most common apoenzymes which needs the cofactor flavin adenine dinucleotide for its activation. The enzyme is generally found in penicillin (Penicillium notatum), hence also called as Notatin.

Carbonic anhydrase

Carbonic anhydrase is a holoenzyme facilitates the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid. The carbonic anhydrase is a metalloenzyme because the apoenzyme form needs coordination with a zing prosthetic group for its activities.

Cytochrome oxidase

Cytochrome oxidase or Cytochrome c oxidase is a transmembrane protein complex found in most of the living organisms including bacteria and archaea also. It is the enzyme present in the respiratory electron transport chain. The complex contains several subunits including two hemes, a cytochrome a and cytochrome a3, and two copper centers, the CuA and CuB centers. The cytochrome oxidase requirs copper ions cofactors for its activation. 

Pyruvate kinase

Pyruvate kinase is an apoenzyme requires Mg2+ ions as the cofactor for its activity. The enzyme required in glycolysis process where it catalyzes the transfer of a phosphate group from phosphoenolpyruvate (PEP) to adenosine diphosphate (ADP), converts one molecule of pyruvate and one molecule of Adenosine triphosphate (ATP). 

Pyruvate carboxylase

Pyruvate carboxylase is another most common apoenzyme which requires Mg2+ ions as the cofactor for its activation just like pyruvate kinase.  The pyruvate carboxylase catalyzes the carboxylation process of pyruvate to form oxaloacetate (OAA). 

Monoamine oxidase

Monoamine oxidase is a catalytic enzyme which catalyzes the oxidative deamination of monoamines where the Oxygen is used to remove an amine group. It is an apoenzyme which is bound with the cofactor FAD for its catalytic activation.


Catalase is another most common example of apoenzymes found in almost every living organism which requires manganese Mg2+ions for its catalytic activation. To protect the cell from oxidative damage, the catalase enzyme breaks the hydrogen peroxide into water and oxygen.

Lactate dehydrogenase

The enzyme lactate dehydrogenase catalyzes the conversion of lactate to pyruvate. It needs the cofactor NAD for its catalytic activation. The enzyme plays a significant role in anaerobic metabolic pathways.

DNA polymerase

DNA polymerase is the primary enzyme that catalyzes the synthesis of DNA molecules from nucleoside triphosphates. The DNA polymerase is a complex enzyme consisting of several subunits. Just like other apoenzyme forms, it also requires manganese (Mg2+) ions for its activities. 

What is important about apoenzymes?

The most important thing about apoenzyme and enzyme is that they are very much temperature and pH sensitive. High temperature or more or less pH level of cellular environment decrease it’s activities, even cause destruction of the enzyme. 

As a whole apoenzymes are one of the most important component catalyzes of biochemical reactions within the cell. Here we discuss about apoenzyme and enzyme along with several important facts related to them. Hope our article about apoenzyme and enzyme will be helpful to you. 

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