Back To CourseAP Biology: Help and Review
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Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science
In this lesson, we'll follow the interactions of two molecules which we'll call Nick and Atty. First, let's learn about Nick.
You may know Nick by many other names like Nicotinic acid, niacin or vitamin B3. Vitamin B3 is niacin. Nick's main role in the body is to produce the twins NADPH and NADH, and without Nick you could end up with pellagra (rash, diarrhea, dementia - a very nasty business). When Nick hangs around with the amides, Nick is more often known as nicotinamide.
When Nick visits cells after being absorbed into them by the stomach, he likes to borrow a ribose and phosphate from phosphoribosyl pyrophosphate, more often called by the acronym PRPP. When Nick does that he's called nicotinate ribonucleotide. This is because with a ribose sugar and phosphate group Nick became a nucleotide, since a nucleotide is a sugar, phosphate, and nitrogen base.
It was as nicotinate ribonucleotide that Nick met Atty. Atty is just what friends call her. You'll probably know Atty better as ATP or adenosine triphosphate. ATP is a very important molecule in the body. It's what the cell utilizes for energy. It is sometimes called molecular currency because it is used in many different processes such as cellular respiration and fermentation. It helps your muscles to work, which lets you run, walk, sit, stand, and breathe.
Once upon a time, Nick and Atty met up in the cell after Nick had borrowed a ribose and phosphate from PRPP and when Atty was waiting around to provide the cell with some extra energy. They ended up having twin cellular derivatives. The twins were given a gift by a passing enzyme, an amide group. These twins had characteristics of both Atty and Nick in his nicotinate ribonucleotide form. The only difference was that one twin had a phosphate group (PO4) instead of an alcohol (OH) group.
They were named after both Nick and Atty. Both were called nicotinamide, since with the added amide group, they looked just like Nick when he was hanging out with the amides. They got an adenine from Atty, so their middle name was adenine, and because both were ultimately composed of 2 nucleotides (one from Atty and one from Nick as nicotinate ribonucleotide) their last names were dinucleotide. Thus, both were called nicotinamide adenine dinucleotide.
But two different molecules can't have the same name, even if they were derived from the same parent molecules. Thus, since one had a phosphate group instead of an alcohol like Atty, that twin had phosphate added to the name, and that's how NADP+ got its name.
So the twin derivatives became known mostly by their acronyms (since their names were long and arduous): NAD+ (nicotinamide adenine dinucleotide) and NADP+ (nicotinamide adenine dinucleotide phosphate). And despite their similar composition, they had very different 3-D structures.
Like many almost identical twins, NAD+ and NADP+ can take each other's place with almost no other cellular component the wiser. Both function as electron carriers. Their main job in the cell is to shuttle electrons around. They also have one more thing they both like to carry, and that's hydrogen. When they carry around hydrogen, they add H to the end of their acronym names. Thus, NAD+ becomes NADH and NADP+ becomes NADPH. The hydrogen is added and removed from their nicotinamide parts. When an H is present, NADH and NADPH are in a reduced form because whenever a molecule receives a hydrogen or electron it's said, in chemistry, to have been reduced. (If they lose a hydrogen or electron, the molecule is oxidized.)
Now despite being very similar, NAD+ and NADP+ are used very differently by the cells. NAD+ has the main job of carrying around electrons for use by the mitochondria and respiration while NADP has a different function. NADPH is used in biosynthesis. In other words, it is used to make biological molecules. NADPH is important in the formation of:
NADPH also can act to reduce cellular oxidants - that is, NADPH can easily give away electrons and hydrogens to other molecules so they take on a reduced form (which prevents them from becoming oxidized). Thus NADPH is a reducing agent (meaning that it has antioxidant properties) that can protect the cell membrane and other cellular structures from becoming oxidized. It functions to reduce glutathione (GSH) and is utilized by cytochrome p450 reductase. On the flip side, NADPH can also be used to produce oxidants, in particular super oxide (O2-) by the enzyme NADPH oxidase (NOX). But oxidants can be used by the cell as a signaling molecule, so it's not all bad.
Speaking of enzymes (proteins that speed up chemical reactions), NADPH also acts as a co-enzyme. This means it helps the enzyme to function and often activates the enzyme. Specifically, it can act as an electron carrier for a group of enzymes known as dehydrogenases that often catalyze oxidation-reduction reactions (or redox reactions).
NADPH is not only important for us, but for plants as well. NADP+ plays a big role as a final electron acceptor in photosynthesis, where it is changed to NADPH. NADPH is the major player involved in the light-independent reactions in chloroplasts where it is converted back to NADP+. In addition, it is known to act as an allosteric regulator for RuBP carboxylase, an important enzyme in photosynthesis.
Often, NADPH can trade places with its twin NADH during nitrogen fixation. Nitrogen fixation is important in the development of fertilizer. Normally NADH acts as a source of electrons for the reduction of dinitrogen (N2), but NADPH can also be used in the same way as NADH in this reaction.
Niacin (vitamin B3) is changed in the cell to nicotinate ribonucleotide, where it combines with ATP and an amide group is added to form NADP+ (nicotinamide adenine dinucleotide phosphate). The addition of a hydrogen creates NADPH, the reduced form of NADP+. NADPH is used in the biosynthesis (production) of lipids (fatty acids and cholesterols), neurotransmitters, nucleotides and amino acids. It also plays a major role in plant photosynthesis as an electron acceptor in the light reaction and donor in the light-independent reactions. In addition to being important in biosynthesis, NADPH can act as an antioxidant, co-enzyme, and source of electrons in nitrogen fixation.
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Back To CourseAP Biology: Help and Review
28 chapters | 382 lessons