6.4
Cofactors and Coenzymes
Enzymes are proteins made of amino acids. The functional group of each constituent amino acid catalyzes a wide variety of chemical reactions via ionic…
Some enzymes associate with non-protein molecules or cofactors to enhance their catalytic activity.
The chemically active enzyme-cofactor complex is called a holoenzyme and the enzyme alone is called an apoenzyme.
Cofactors can be present as inorganic metal ions such as the zinc ion in carbonic anhydrase or as organic molecules called coenzymes such as NAD+, coenzyme A and others.
Some cofactors, called prosthetic groups, are covalently bonded to apoenzymes. For example, in hemoglobin, the heme group, a porphyrin with a central iron ion, is associated with protein chains through covalent bonding, hydrogen bonding, and extensive hydrophobic interactions.
In contrast, cofactors that are transiently bound to enzymes act as cosubstrates.
For example, NAD+ , a coenzyme for alcohol dehydrogenase, helps oxidize alcohol to aldehyde and gets reduced to NADH. NADH dissociates once the reaction is complete and is reoxidized to NAD+ for the next reaction cycle.
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Q1: What is the difference between a holoenzyme and an apoenzyme?
An apoenzyme is an enzyme protein alone without cofactors. A holoenzyme is the chemically active complex formed when an apoenzyme associates with non-protein cofactors that enhance its catalytic activity. Cofactors are essential because amino acids cannot catalyze oxidation-reduction and group transfer reactions independently.
Q2: What types of molecules can serve as cofactors?
Cofactors can be inorganic metal ions like zinc, magnesium, iron, copper, cobalt, or manganese, or organic molecules called coenzymes such as NAD+, coenzyme A, and biotin. Many vitamins function as coenzymes, including derivatives of vitamin B3 and vitamin A. Cofactors are present in approximately 30% of mature proteins and are frequently incorporated during protein folding to enable ligand binding sites and protein ligand interactions.
Q3: How do prosthetic groups differ from cosubstrates?
Prosthetic groups are cofactors covalently bonded to apoenzymes through covalent bonds, hydrogen bonding, and hydrophobic interactions, remaining permanently attached. Cosubstrates are cofactors transiently bound to enzymes that participate in reactions and dissociate afterward. For example, NAD+ acts as a cosubstrate for alcohol dehydrogenase, becoming reduced to NADH and then released.
Q4: Why do enzymes need cofactors to catalyze certain reactions?
Amino acids in enzymes can catalyze ionic interactions and acid-base reactions but cannot catalyze oxidation-reduction and group transfer reactions alone. Cofactors, called the chemical teeth of enzymes, provide the additional chemical capability needed for these complex reactions. This is why cofactors are frequently incorporated into enzymes during protein folding.
Q5: What role does magnesium play as a cofactor in DNA synthesis?
Magnesium is an essential cofactor for over 300 enzymes in the human body, including DNA polymerase. The magnesium ion helps form the phosphodiester bond on the DNA backbone during DNA synthesis. This demonstrates how metal ion cofactors directly participate in critical cellular processes.
Q6: Why must certain cofactors like NAD+ and vitamin A be obtained from diet?
Many cofactors, including NAD+ derived from vitamin B3 and retinol from vitamin A, cannot be synthesized by human cells and must be ingested through diet. These coenzymes are essential for various enzymatic reactions throughout the body. Biotin, another B vitamin coenzyme, is critical for enzymes that transfer carbon dioxide between molecules.
Q7: How does the heme group function as a prosthetic group in hemoglobin?
The heme group, a porphyrin with a central iron ion, is associated with hemoglobin protein chains through covalent bonding, hydrogen bonding, and extensive hydrophobic interactions. As a prosthetic group, the heme remains permanently attached to the protein. This stable association enables hemoglobin to bind and transport oxygen effectively throughout the body.