Holoenzyme vs. Apoenzyme — What's the Difference?
By Fiza Rafique & Maham Liaqat — Updated on May 8, 2024
A holoenzyme is a complete, active enzyme with its cofactor, while an apoenzyme is the inactive protein portion of the enzyme without its cofactor.
Difference Between Holoenzyme and Apoenzyme
Table of Contents
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Key Differences
Holoenzyme refers to a fully assembled enzyme complex, including the protein portion and the required cofactor, making it catalytically active. Apoenzyme, on the other hand, is just the protein part of an enzyme and remains inactive without its necessary cofactor.
Holoenzymes can carry out their biological function, catalyzing chemical reactions due to the cofactor's presence. Apoenzymes are non-functional until their specific cofactor is bound.
Holoenzyme activity relies on cofactors, which can be organic molecules like vitamins or inorganic ions. Apoenzymes must acquire these cofactors through binding to become functional.
Holoenzyme formation is essential for many metabolic processes in cells. Apoenzymes, meanwhile, form the structural framework that determines enzyme specificity and function once activated.
Comparison Chart
Definition
Active enzyme with cofactor
Inactive enzyme without cofactor
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Functionality
Fully functional in catalysis
Inactive until cofactor binds
Composition
Protein and cofactor
Protein only
Cofactor
Bound and necessary for activity
Missing or not bound
Role
Essential in metabolic processes
Structural framework awaiting activation
Compare with Definitions
Holoenzyme
Fully active due to cofactor binding.
RNA polymerase becomes a holoenzyme after binding sigma factor.
Apoenzyme
The protein component of an enzyme.
The apoenzyme of lactate dehydrogenase lacks its NAD+ cofactor.
Holoenzyme
Often essential for cellular metabolism.
Pyruvate dehydrogenase acts as a holoenzyme in cellular respiration.
Apoenzyme
Inactive until a cofactor is bound.
Without zinc ions, the carbonic anhydrase apoenzyme is inactive.
Holoenzyme
Cofactor can be organic or inorganic.
Iron is the cofactor for the catalase holoenzyme.
Apoenzyme
Serves as the enzyme's structural framework.
The protein structure of an apoenzyme determines substrate specificity.
Holoenzyme
Catalytically efficient due to cofactor presence.
Coenzyme A enables the holoenzyme to facilitate acyl group transfers.
Apoenzyme
Cofactors needed for the catalytic mechanism.
Vitamin B6 is a cofactor necessary to activate the transaminase apoenzyme.
Holoenzyme
A complete enzyme complex containing a cofactor.
DNA polymerase functions as a holoenzyme for DNA replication.
Apoenzyme
Binds cofactors to form an active enzyme.
ATP synthase becomes functional only when its F1 unit binds cofactors.
Holoenzyme
An active, complex enzyme consisting of an apoenzyme and a coenzyme.
Apoenzyme
The protein component of an enzyme, excluding additional components such as cofactors or inhibitors.
Holoenzyme
(enzyme) A haloenzyme.
Apoenzyme
(enzyme) An inactive haloenzyme lacking a cofactor.
Apoenzyme
A protein that combines with a coenzyme to form an active enzyme
Common Curiosities
Is a holoenzyme always active?
Yes, a holoenzyme is active due to the presence of its cofactor.
Can an apoenzyme bind different cofactors?
Typically, each apoenzyme requires a specific cofactor for activation.
Are cofactors always inorganic?
No, cofactors can be organic molecules (coenzymes) or inorganic ions.
What role does a cofactor play in a holoenzyme?
Cofactors often aid in substrate binding or the catalytic process.
Can an apoenzyme still bind substrates?
It may bind substrates, but cannot catalyze reactions without a cofactor.
Are all enzymes holoenzymes?
No, only those requiring a cofactor for activity are considered holoenzymes.
Is it easy to separate a cofactor from a holoenzyme?
Not always; the binding can be tight or specific.
Are apoenzymes stable without cofactors?
Yes, but they remain inactive.
How does an apoenzyme differ from a coenzyme?
An apoenzyme is the inactive protein, while a coenzyme is an organic cofactor.
Can an apoenzyme catalyze reactions?
No, an apoenzyme lacks catalytic activity without its cofactor.
Is it possible to have a holoenzyme without an apoenzyme?
No, a holoenzyme must include an apoenzyme as its protein component.
Do all holoenzymes contain metal ions as cofactors?
No, some have organic molecules like vitamins as cofactors.
What are some examples of holoenzymes?
Examples include RNA polymerase and DNA polymerase.
Can a holoenzyme lose its activity?
Yes, if it loses its cofactor, it becomes inactive.
Do all enzymes require cofactors to function?
No, some enzymes can function independently without a cofactor.
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Written by
Fiza RafiqueFiza Rafique is a skilled content writer at AskDifference.com, where she meticulously refines and enhances written pieces. Drawing from her vast editorial expertise, Fiza ensures clarity, accuracy, and precision in every article. Passionate about language, she continually seeks to elevate the quality of content for readers worldwide.
Co-written by
Maham Liaqat
