If you have done any research on aging and health recently, you have likely stumbled across the so-called anti-aging molecule, NAD. You have probably also seen it called NAD+ and maybe even as NADH. So, what is the difference, if there is any?
The short answer is that there is a difference, at least between NAD and NADH. Generally speaking, when NAD is used NAD is being talked about generally. And often when using “NAD” it is referring to the specific chemical forms of NAD, NAD+ and NADH, interchangeably.
NAD+ is written with a superscript plus (+) to designate the molecule’s charge and its specific chemical state. NADH is referring to the specific opposite chemical state that NAD can be found within your cells. A more detailed explanation is one that requires a jog down memory lane, back to the days of chemistry (more on that later).
NAD stands for nicotinamide adenine dinucleotide, which is just a fancy name describing the parts of its chemical structure. It is a molecule found in all living cells that is essential for metabolism and the proper functioning of many other key molecules as mentioned above. NAD exists in two forms: NAD+ and NADH. Its ability to switch between these two forms is what allows NAD to carry out its main function—carrying electrons from one reaction to another in the process of metabolism and energy production.
As an electron carrier, NAD+ and NADH help to convert the nutrients in your food into a form of energy your cells can use. Here’s how the process works, beginning with a quick refresher on some basic chemistry.
The difference between NAD+ and NADH is two electrons and a hydrogen
As you probably remember, atoms are the smallest unit of matter and molecules are just a collection of atoms held together by chemical bonds. NAD+ and NADH are considered molecules, containing the atoms carbon, hydrogen, nitrogen, oxygen, and phosphorous.
Atoms are made up of particles called protons, electrons, and neutrons. Protons carry a positive electrical charge, electrons carry a negative charge, and neutrons carry no charge. Atoms are generally neutral particles with the number of protons and electrons being equal.
The protons and neutrons are located at the center of an atom, called the nucleus. The electrons orbit around the nucleus in what are called shells or orbitals. The positively and negatively charged particles act like magnets, which is what keeps the electrons bound to the nucleus of the atom.
Ideally, an atom likes to have eight electrons in its outer shell—what is called the octet rule. Atoms are most stable when their outer shells are full and the charges are balanced. When they have fewer than eight electrons, or the charges become unequal, they become reactive. This is one reason chemical reactions take place.
In order to achieve a stable state, atoms will share their electrons. This results in chemical bonds and enables the formation of molecules, such as NAD. In its most stable state, NAD is positively charged (hence, the name NAD+). The reason being that when all the atoms making up the molecule bond together, one of the nitrogen atoms ends up with an unequal number of electrons and protons.
Remember, on their own, atoms are neutral because they have an equal number of protons and electrons. In this case, the nitrogen atom ends up with one more proton than electrons, giving the molecule a positive charge.
NADH happens when NAD+ accepts a hydride atom—a hydrogen atom with an extra electron or two electrons total (H–). From the hydride electron pair, one electron is transferred to the positively charged nitrogen of the nicotinamide ring of NAD+, and the second hydrogen atom transferred to the C4 carbon atom opposite the nitrogen atom. The reaction is easily reversible when NADH reduces another molecule and is converted back to NAD+. This means the coenzyme can continuously cycle between the NAD+ and NADH forms without being consumed in the process. This is exactly the power of NAD.
NAD’s role in metabolism involves giving and taking electrons
Chemical reactions will also occur if new molecules are introduced into the system, as is the case when you eat. The carbohydrates, fats, and proteins in the food you eat are all just a collection of atoms. Metabolism is the process of breaking these large molecules (often called macromolecules) into their component parts so they can be used as energy or as building blocks for cellular structures.
The chemical reactions associated with metabolism include a series of steps whereby one molecule is transformed into another molecule. This occurs as a result of redox reactions (also called oxidation-reduction reactions), which involve the transfer of electrons between molecules.
Each step is facilitated by a specific enzyme, molecules that help to accelerate chemical reactions. Oxidoreductase is the enzyme that initiates the transfer of electrons from one molecule, also called the electron donor, to another, called the electron acceptor. This group of enzymes typically uses cofactors, such as NAD, which acts as the electron acceptor. The food molecule acts as the electron donor.
Due to its chemical structure, each molecule of NAD+ can accept two electrons. This gain of electrons is called reduction, with the electrons coming in the form of a hydrogen atom. In a redox reaction, the hydrogen atom contains two electrons which it shares with the NAD+ molecule. The bond that is formed between NAD+ and H– is what creates NADH, the other form of NAD.
NADH is considered the activated carrier molecule. It acts to transfer these extra electrons to the inner membrane of the mitochondria where they are donated to a structure called the electron transport chain. Like the food molecule, NADH functions as an electron donor.
The electron transporters embedded in the mitochondrial membrane are oxidoreductases that shuttle electrons from NADH to molecular oxygen, another electron acceptor. This loss of electrons is called oxidation. NADH undergoes a reverse reaction, converting back to NAD+.
The process of electron transfer is coupled with the movement of protons, in the form of H+ ions, across the inner membrane. This pumping of positive charges from one side of the membrane to the other activates the protein responsible for generating ATP, the fuel used by your cells. The NAD+ that is leftover can then be reused as an electron acceptor as more food enters the system.
NAD also has other essential functions in the cell
For example, from Wikipedia:
NAD can also activate a number of other essential enzymes in the cell.
The coenzyme NAD+ is also consumed in ADP-ribose transfer reactions. For example, enzymes called ADP-ribosyltransferases add the ADP-ribose moiety of this molecule to proteins, in a posttranslational modification called ADP-ribosylation. ADP-ribosylation involves either the addition of a single ADP-ribose moiety, in mono-ADP-ribosylation, or the transferral of ADP-ribose to proteins in long branched chains, which is called poly(ADP-ribosyl)ation. Mono-ADP-ribosylation was first identified as the mechanism of a group of bacterial toxins, notably cholera toxin, but it is also involved in normal cell signaling. Poly(ADP-ribosyl)ation is carried out by the poly(ADP-ribose) polymerases. The poly(ADP-ribose) structure is involved in the regulation of several cellular events and is most important in the cell nucleus, in processes such as DNA repair and telomere maintenance. In addition to these functions within the cell, a group of extracellular ADP-ribosyltransferases has recently been discovered, but their functions remain obscure. NAD+ may also be added onto cellular RNA as a 5′-terminal modification.
Another function of this coenzyme in cell signaling is as a precursor of cyclic ADP-ribose, which is produced from NAD+ by ADP-ribosyl cyclases, as part of a second messenger system. This molecule acts in calcium signaling by releasing calcium from intracellular stores. It does this by binding to and opening a class of calcium channels called ryanodine receptors, which are located in the membranes of organelles, such as the endoplasmic reticulum.
NAD+ is also consumed by sirtuins, which are NAD-dependent deacetylases, such as Sir2. These enzymes act by transferring an acetyl group from their substrate protein to the ADP-ribose moiety of NAD+; this cleaves the coenzyme and releases nicotinamide and O-acetyl-ADP-ribose. The sirtuins mainly seem to be involved in regulating transcription through deacetylating histones and altering nucleosome structure. However, non-histone proteins can be deacetylated by sirtuins as well. These activities of sirtuins are particularly interesting because of their importance in the regulation of aging.
Other NAD-dependent enzymes include bacterial DNA ligases, which join two DNA ends by using NAD+ as a substrate to donate an adenosine monophosphate (AMP) moiety to the 5′ phosphate of one DNA end. This intermediate is then attacked by the 3′ hydroxyl group of the other DNA end, forming a new phosphodiester bond. This contrasts with eukaryotic DNA ligases, which use ATP to form the DNA-AMP intermediate.
Li et al. have found that NAD+ directly regulates protein-protein interactions. They also show that one of the causes of age-related decline in DNA repair may be increased binding of the protein DBC1 (Deleted in Breast Cancer 1) to PARP1 (poly[ADP–ribose] polymerase 1) as NAD+ levels decline during aging. Thus, the modulation of NAD+ may protect against cancer, radiation, and aging.
NAD can also function as a cell-signaling molecule
In recent years, NAD+ has also been recognized as an extracellular signaling molecule involved in cell-to-cell communication. NAD+ is released from neurons in blood vessels,urinary bladder,large intestine, from neurosecretory cells, and from brain synaptosomes, and is proposed to be a novel neurotransmitter that transmits information from nerves to effector cells in smooth muscle organs. In plants, the extracellular nicotinamide adenine dinucleotide induces resistance to pathogen infection and the first extracellular NAD receptor has been identified. Further studies are needed to determine the underlying mechanisms of its extracellular actions and their importance for human health and life processes in other organisms.
NAD is a Dynamic Molecule
Many biological processes are dedicated to breaking down molecules into their component atoms so they can be reassembled into other useful molecules. Metabolism is one such process that functions to convert food into energy as well as building blocks for cell structures. Because its end products are vital to many cell functions, it is often referred to as the set of life-sustaining chemical reactions.
Part of the metabolic process involves transferring electrons between molecules. This transfer of electrons occurs as a result of redox reactions, whereby one molecule donates electrons and another molecule accepts electrons. NAD is one of the main electron carriers in redox reactions, with a unique ability to function as both a donor and an acceptor.
To perform its role as an electron carrier, NAD reverts back and forth between two forms, NAD+ and NADH. NAD+ accepts electrons from food molecules, transforming it into NADH. NADH donates electrons to oxygen, converting it back to NAD+.
The relative proportion of these two molecules depends on the energy state of the cell, with more NADH being present in a fed state. The NAD+:NADH ratio can act as a signal, alerting the cell to changes in its energy status. This signaling mechanism is believed to be important for the activation of a number of cellular enzymes essential to adaptive cellular responses that function to maintain cellular health.