What is Nicotinamide Adenine Dinucleotide (NAD)?

Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

In this lesson you will discover what nicotinamide adenine dinucleotide is and what it looks like. You will also learn about its important functions in the cell and then you'll be able to test your knowledge with a quiz!

Importance of Nicotinamide Adenine Dinucleotide

Money: in this world, dollars and cents are important forms of currency that you can exchange for something you want. If you want a hamburger you have to pay a certain amount of dollars and cents for the hamburger.

In the cell, the currency is not dollars and cents, but molecules. In particular, for respiration and energy, the currency is electrons and hydrogen. In order to produce energy the cell has to use electrons and hydrogen. But how does the cell get the electrons and hydrogen it needs to make and store energy?

That is where the electron carriers come in, the carriers of cellular money. One of these carriers is called nicotinamide adenine dinculeotide or NAD+ for short. NAD+ is an important conveyor of hydrogen and electrons for the cell and is essential for the continued production of energy (ATP) by the mitochondria.

Structure of NAD+

The vitamin B3 is used by the cell to form part of the molecule NAD+.

Vitamin B3
vitamin B3 structure

ATP, which is scientifically known as adenosine triphosphate, is used to form another portion of the molecule.

ATP Structure
ATP structure

The important part of NAD+, the part that holds on to cellular currency, comes from the part of the molecule formed from vitamin B3. NAD+ is able to accept and carry hydrogen and two electrons.

Structure of NAD+

When it is acting as a cellular money carrier, the name changes from NAD+ to NADH, which is the reduced form of nicotinamide adenine dinucleotide, because of the extra H attached.

NADH Structure
NADH structure

Function of NAD+/NADH

NAD+ acts as an electron and hydrogen acceptor and NADH acts as a hydrogen and electron donor. You can think of it as NAD+ takes the money and NADH gives it away. So when does NAD+ take money and when does NADH give it away, and why does it give it away?

Let's look first at why NADH gives away the money it collects. The cell needs energy to survive and, in order to make energy, the hydrogen and electrons that NADH give away are needed. So NADH is important in cellular respiration, which is where energy is produced. NADH gives two electrons to the electron transport chain (ETC) and allows the hydrogen to be used to help move the electrons along the ETC. There are different clumps of molecules called complexes that move electrons and hydrogen. The first one is called Complex I. It is at this complex that NADH gives away 2 electrons and a hydrogen.

NADH gives hydrogen and electrons to Complex I.
NADH gives H and e- to Complex I

In cases where NADH cannot get to the ETC, it can be used differently. During fermentation it is used to produce ethanol, and can be used to form lactic acid in low oxygen conditions.

So now we know where and why NADH gives away the money, but how does it get the money to give away? This is where NAD+ comes in.

During the process of glycolysis, where the sugar glucose is broken down, energy is released in the form of electrons. Without anything there to accept these electrons, the released energy would probably just end up as heat. One glucose molecule breaks down into two pyruvate molecules. In this reaction NAD+ accepts electrons and hydrogen, thus two molecules of NADH are formed for every one molecule of glucose.

Glycolysis produces 2 NADH and 2 pyruvate

The two pyruvate molecules are further changed and made to form two acetyl-CoA, a process called pyruvate oxidation. During this process more electrons and hydrogen are collected by NAD+.

Pyruvate oxidation produces NADH
pyruvate oxidation

Acetyl-CoA is a very important molecule for the cell and can be used by a variety of pathways. One main pathway is the citric acid cycle or Kreb's cycle. For every turn of the citric acid cycle, 3 NAD+ collect electrons and form 3 NADH. If both acetyl-CoA's formed from pyruvate oxidation go through the citric acid cycle, the cell can end up with 6 NADH.

The citric acid cycle can produce 3 NADH molecules.
citric acid cycle produces NADH

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