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. There is no sharp division between loosely and tightly bound cofactors. Other coenzymes,
flavin adenine dinucleotide (FAD),
biotin, and
lipoamide, for instance, are tightly 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. Although enzyme catalyzed industrial processes are highly efficient, some of the enzymes are dependent on nicotinamide cofactors (NADH/NAD+, NADP/NAPH). Due to the high price of such cofactors, these processes would not be economically competitive. Recently, some synthetic organic compounds were identified as economically promising biomimetic counterparts of natural cofactors.
Vitamins and derivatives Non-vitamins Cofactors as metabolic intermediates reactions of
nicotinamide adenine dinucleotide. Metabolism involves a vast array of chemical reactions, but most fall under a few basic types of reactions that involve the transfer of
functional groups. This common chemistry allows cells to use a small set of metabolic intermediates to carry chemical groups between different reactions. These group-transfer intermediates are the loosely bound organic cofactors, often called
coenzymes. Each class of group-transfer reaction is carried out by a particular cofactor, which is the substrate for a set of enzymes that produce it, and a set of enzymes that consume it. An example of this are the
dehydrogenases that use
nicotinamide adenine dinucleotide (NAD+) as a cofactor. Here, hundreds of separate types of enzymes remove electrons from their substrates and
reduce NAD+ to NADH. This reduced cofactor is then a substrate for any of the
reductases in the cell that require electrons to reduce their substrates. Therefore, these cofactors are continuously recycled as part of
metabolism. As an example, the total quantity of ATP in the human body is about 0.1
mole. This ATP is constantly being broken down into ADP, and then converted back into ATP. Thus, at any given time, the total amount of ATP + ADP remains fairly constant. The energy used by human cells requires the
hydrolysis of 100 to 150 moles of ATP daily, which is around 50 to 75 kg. In typical situations, humans use up their body weight of ATP over the course of the day. This means that each ATP molecule is recycled 1000 to 1500 times daily.
Evolution Organic cofactors, such as
ATP and
NADH, are present in all known forms of life and form a core part of
metabolism. Such universal
conservation indicates that these molecules evolved very early in the development of living things. At least some of the current set of cofactors may, therefore, have been present in the
last universal ancestor, which lived about 4 billion years ago. Organic cofactors may have been present even earlier in the
history of life on Earth. The nucleotide
adenosine is a cofactor for many basic metabolic enzymes such as transferases. It may be a remnant of the
RNA world. Adenosine-based cofactors may have acted as adaptors that allowed enzymes and ribozymes to bind new cofactors through small modifications in existing adenosine-binding
domains, which had originally evolved to bind a different cofactor. This process of adapting a pre-evolved structure for a novel use is known as
exaptation.
Prebiotic origin of coenzymes. Like
amino acids and
nucleotides, certain
vitamins and thus coenzymes can be created under early earth conditions. For instance,
vitamin B3 can be synthesized with electric discharges applied to
ethylene and
ammonia. Similarly,
pantetheine (a
vitamin B5 derivative), a precursor of
coenzyme A and thioester-dependent synthesis, can be formed spontaneously under evaporative conditions. Other coenzymes may have existed early on Earth, such as
pterins (a derivative of
vitamin B9),
flavins (
FAD,
flavin mononucleotide = FMN), and
riboflavin (vitamin B2).
Changes in coenzymes. A computational method, IPRO, recently predicted mutations that experimentally switched the cofactor specificity of
Candida boidinii xylose reductase from NADPH to NADH.
Evolution of enzymes without coenzymes. If enzymes require a co-enzyme, how does the coenzyme evolve? The most likely scenario is that enzymes can function initially without their coenzymes and later recruit the coenzyme, even if the catalyzed reaction may not be as efficient or as fast. Examples are
Alcohol Dehydrogenase (coenzyme:
NAD⁺),
Lactate Dehydrogenase (NAD⁺),
Glutathione Reductase (
NADPH).
History The first organic cofactor to be discovered was NAD+, which was identified by
Arthur Harden and William Young 1906. They noticed that adding boiled and filtered
yeast extract greatly accelerated
alcoholic fermentation in unboiled yeast extracts. They called the unidentified factor responsible for this effect a
coferment. Through a long and difficult purification from yeast extracts, this heat-stable factor was identified as a
nucleotide sugar phosphate by
Hans von Euler-Chelpin. Other cofactors were identified throughout the early 20th century, with ATP being isolated in 1929 by Karl Lohmann, and coenzyme A being discovered in 1945 by
Fritz Albert Lipmann. The functions of these molecules were at first mysterious, but, in 1936,
Otto Heinrich Warburg identified the function of NAD+ in hydride transfer. This discovery was followed in the early 1940s by the work of
Herman Kalckar, who established the link between the oxidation of sugars and the generation of ATP. This confirmed the central role of ATP in energy transfer that had been proposed by Fritz Albert Lipmann in 1941. Later, in 1949, Morris Friedkin and
Albert L. Lehninger proved that NAD+ linked metabolic pathways such as the citric acid cycle and the synthesis of ATP. ==Protein-derived cofactors==