Question: What can be an example for an organic cofactor?
About the article "Biomolecule", section "Apoenzymes".

Candidate passages:
 and coenzymes.
Some enzymes do not need any additional components to show full activity.
However, others require non-protein molecules called cofactors to be bound for activity.
Cofactors can be either inorganic (e.g., metal ions and iron-sulfur clusters) or organic compounds (e.g., flavin and heme).
Organic cofactors can be either prosthetic groups, which are tightly bound to an enzyme, or coenzymes, which are released from the enzyme's active site during the reaction.
Coenzymes include NADH, NADPH and adenosine triphosphate.
These molecules transfer chemical groups between enzymes.
An example of an enzyme that contains a cofactor is carbonic anhydrase, and is shown in the ribbon diagram above with a zinc cofactor bound as part of its active site.
These tightly bound molecules are usually found in the active site and are involved in catalysis.
For example, flavin and heme cofactors are often involved in redox reactions.
Enzymes that require a cofactor but do not have one bound are called "apoenzymes" or "apoproteins".
An apoenzyme together with its cofactor(s) is called a "holoenzyme" (this is the active form).
Most cofactors are not covalently attached to an enzyme, but are very tightly bound.
However, organic prosthetic groups can be covalently bound (e.g., biotin in the enzyme pyruvate carboxylase).
The term "holoenzyme" can also be applied to enzymes that contain multiple protein subunits, such as the DNA polymerases; here the holoenzyme is the complete complex containing all the subunits needed for activity. 
A cofactor is a non-protein chemical compound that is required for the protein's biological activity.
These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations.
Cofactors can be classified depending on how tightly they bind to an enzyme, with loosely bound cofactors termed coenzymes and tightly bound cofactors termed prosthetic groups.
Some sources also limit the use of the term "cofactor" to inorganic substances.
An inactive enzyme, without the cofactor is called an apoenzyme, while the complete enzyme with cofactor is the holoenzyme.
Some enzymes or enzyme complexes require several cofactors.
For example, the multienzyme complex pyruvate dehydrogenase at the junction of glycolysis and the citric acid cycle requires five organic cofactors and one metal ion: loosely bound thiamine pyrophosphate (TPP), covalently bound lipoamide and flavin adenine dinucleotide (FAD), and the cosubstrates nicotinamide adenine dinucleotide (NAD+) and coenzyme A (CoA), and a metal ion (Mg2+).
Organic cofactors are often vitamins or are made from vitamins.
Many contain the nucleotide adenosine monophosphate (AMP) as part of their structures, such as ATP, coenzyme A, FAD, and NAD+.
This common structure may reflect a common evolutionary origin as part of ribozymes in an ancient RNA world.
It has been suggested that the AMP part of the molecule can be considered a kind of "handle" by which the enzyme can "grasp" the coenzyme to switch it between different catalytic centers. 
Organic cofactors are small organic molecules (typically a molecular mass less than 1000 Da) that can be either loosely or tightly bound to the enzyme and directly participate in the reaction.
In the latter case, when it is difficult to remove without denaturing the enzyme, it can be called a prosthetic group.
It is important to emphasize that there is no sharp division between loosely and tightly bound cofactors.
Indeed, many such as NAD+ can be tightly bound in some enzymes, while it is loosely bound in others.
Another example is thiamine pyrophosphate (TPP), which is tightly bound in transketolase or pyruvate decarboxylase, while it is less tightly bound in pyruvate dehydrogenase.
Other coenzymes, flavin adenine dinucleotide (FAD), biotin, and lipoamide, for instance, are covalently bound.
Tightly bound cofactors are, in general, regenerated during the same reaction cycle, while loosely bound cofactors can be regenerated in a subsequent reaction catalyzed by a different enzyme.
In the latter case, the cofactor can also be considered a substrate or cosubstrate.
Vitamins can serve as precursors to many organic cofactors (e.g., vitamins B1, B2, B6, B12, niacin, folic acid) or as coenzymes themselves (e.g., vitamin C).
However, vitamins do have other functions in the body.
Many organic cofactors also contain a nucleotide, such as the electron carriers NAD and FAD, and coenzyme A, which carries acyl groups.
Most of these cofactors are found in a huge variety of species, and some are universal to all forms of life.
An exception to this wide distribution is a group of unique cofactors that evolved in methanogens, which are restricted to this group of archaea. 
An apoenzyme (or, generally, an apoprotein) is the protein without any small-molecule cofactors, substrates, or inhibitors bound.
It is often important as an inactive storage, transport, or secretory form of a protein.
This is required, for instance, to protect the secretory cell from the activity of that protein.
Apoenzymes becomes active enzymes on addition of a cofactor.
Cofactors can be either inorganic (e.g., metal ions and iron-sulfur clusters) or organic compounds, (e.g., flavin and heme).
Organic cofactors can be either prosthetic groups, which are tightly bound to an enzyme, or coenzymes, which are released from the enzyme's active site during the reaction.
Cofactors were discovered by Arthur Harden and William Young in 1906, when they found that the rate of alcoholic fermentation in unboiled yeast extracts increased when boiled yeast extract was added.
A few years after, Hans von Euler-Chelpin identified the cofactor in the boiled extract as NAD+.
Other cofactors, such as ATP and coenzyme A, were discovered later in the 1900s.
The mechanism of cofactor activity was discovered when, Otto Heinrich Warburg determined in 1936 that NAD+ functioned as an electron acceptor.
Well after these initial discoveries, scientists began to realize that the manipulation of cofactor concentrations could be used as tools for the improvement of metabolic pathways.
An important group of organic cofactors is the family of molecules referred to as vitamins.
Vitamin B12 (cobalamin), for example, plays a crucial role in the human body, while coenzyme B12, its derivative, is found in the metabolisms of every type of cell in our bodies.
Its presence affects the synthesis and regulation of cellular DNA as well as taking part in fatty acid synthesis and energy production.
Cofactors are required by many important metabolic pathways, and it is possible for the concentrations of a single type of cofactor to affect the fluxes of many different pathways .
Minerals and metallic ions that organisms uptake through their diet provide prime examples of inorganic cofactors.
For instance Zn2+ is needed to assist the enzyme carbonic anhydrase as it converts carbon dioxide and water to bicarbonate and protons.
A widely recognized mineral that acts as a cofactor is iron, which is essential for the proper function of hemoglobin, the oxygen transporting protein found in red blood cells.
This example in particular highlights the importance of cofactors in animal metabolism.