Question

Explain the roles of ATP and NADPH in catabolic and anabolic reactions.

 

Answer 1

ATP

Catabolic reactions break down large organic molecules into smaller molecules, releasing the energy contained in the chemical bonds. These energy releases (conversions) are not 100 percent efficient. The amount of energy released is less than the total amount contained in the molecule. Approximately 40 percent of energy yielded from catabolic reactions is directly transferred to the high-energy molecule adenosine triphosphate (ATP). ATP, the energy currency of cells, can be used immediately to power molecular machines that support cell, tissue, and organ function. This includes building new tissue and repairing damaged tissue. ATP can also be stored to fulfill future energy demands. The remaining 60 percent of the energy released from catabolic reactions is given off as heat, which tissues and body fluids absorb.

Structurally, ATP molecules consist of an adenine, a ribose, and three phosphate groups (Figure 1). The chemical bond between the second and third phosphate groups, termed a high-energy bond, represents the greatest source of energy in a cell. It is the first bond that catabolic enzymes break when cells require energy to do work. The products of this reaction are a molecule of adenosine diphosphate (ADP) and a lone phosphate group (Pi). ATP, ADP, and Pi are constantly being cycled through reactions that build ATP and store energy, and reactions that break down ATP and release energy.

Figure 1. Structure of ATP Molecule. Adenosine triphosphate (ATP) is the energy molecule of the cell. During catabolic reactions, ATP is created and energy is stored until needed during anabolic reactions.

The energy from ATP drives all bodily functions, such as contracting muscles, maintaining the electrical potential of nerve cells, and absorbing food in the gastrointestinal tract. The metabolic reactions that produce ATP come from various sources (Figure 2).

Figure 2. Sources of ATP. During catabolic reactions, proteins are broken down into amino acids, lipids are broken down into fatty acids, and polysaccharides are broken down into monosaccharides. These building blocks are then used for the synthesis of molecules in anabolic reactions.

Of the four major macromolecular groups (carbohydrates, lipids, proteins, and nucleic acids) that are processed by digestion, carbohydrates are considered the most common source of energy to fuel the body. They take the form of either complex carbohydrates, polysaccharides like starch and glycogen, or simple sugars (monosaccharides) like glucose and fructose. Sugar catabolism breaks polysaccharides down into their individual monosaccharides. Among the monosaccharides, glucose is the most common fuel for ATP production in cells, and as such, there are a number of endocrine control mechanisms to regulate glucose concentration in the bloodstream. Excess glucose is either stored as an energy reserve in the liver and skeletal muscles as the complex polymer glycogen, or it is converted into fat (triglyceride) in adipose cells (adipocytes).

Among the lipids (fats), triglycerides are most often used for energy via a metabolic process called β-oxidation. About one-half of excess fat is stored in adipocytes that accumulate in the subcutaneous tissue under the skin, whereas the rest is stored in adipocytes in other tissues and organs.

Proteins, which are polymers, can be broken down into their monomers, individual amino acids. Amino acids can be used as building blocks of new proteins or broken down further for the production of ATP. When one is chronically starving, this use of amino acids for energy production can lead to a wasting away of the body, as more and more proteins are broken down.

Nucleic acids are present in most of the foods you eat. During digestion, nucleic acids including DNA and various RNAs are broken down into their constituent nucleotides. These nucleotides are readily absorbed and transported throughout the body to be used by individual cells during nucleic acid metabolism.

ANABOLIC REACTIONS

In contrast to catabolic reactions, anabolic reactions involve the joining of smaller molecules into larger ones. Anabolic reactions combine monosaccharides to form polysaccharides, fatty acids to form triglycerides, amino acids to form proteins, and nucleotides to form nucleic acids. These processes require energy in the form of ATP molecules generated by catabolic reactions. Anabolic reactions, also called biosynthesis reactions, create new molecules that form new cells and tissues, and revitalize organs.

NADPH

NADPH and its role in anabolism (making larger molecules out of smaller ones).

Oxidation-Reduction Reactions need "reducing agents"

ATP is used in everything from cellular "housekeeping" to the movements of muscle cells to the molecule-building of anabolism. However, the chemical reactions of anabolism often require something that ATP doesn't provide called "reducing agents".

1. 2Na + Cl2
→

2. 2Na+ + 2e- + Cl2
→

3. 2Na+ + 2Cl-
→

4. 2NaCl

This is a simple redox reaction.
In (2.), the 2Na has given up
two electrons to become 2Na+.
In (3.), the Cl2 has picked up
two electrons to become 2Cl-.
In (4.), the 2Na+ and the 2Cl-
have bonded to form 2NaCl.

"Reduction" is when a molecule, atom or ion loses at least some control over electron(s). "Oxidation" is when a molecule, atom or ion gains at least some control over electrons.

For either to happen, the reduction and oxidation need to happen
at the same time and together they are called an
"oxidation-reduction reaction" ("redox reactions" is the nickname their friends use).

In the example on the right, electrons are lost by sodium atoms and picked up by chlorine atoms.

However, you can also have an oxidation-reduction reaction where electrons are not completely transferred and one substance just has less of a "share" of its electron(s) than it used to (this is what happens in redox reactions that break and form covalent bonds - see oxidation-reduction).

In these sorts of chemical reactions, a substance that gives up an electron (gets itself "oxidized) is called a "reducing agent" and one that gains an electron (gets itself "reduced") is called the "oxidizing agent".

The reaction that forms a molecule
of the fatty acid palmitate. It is
worth noting that ATP was used in
the formation of malonyl-CoA2. (3)

NADPH - everything reduced!

NADPH molecules are created in catabolism when a negative hydride anion is bonded to a molecule of NADP+. A "hydride anion" (H-) is a hydrogen atom with an extra electron (two e- instead of one e-) and therefore a negative charge.

When used as a reducing agent, each NADPH molecule gives up the hydride anion (H-), providing two electrons (2 e-) to help move the reaction forward. In the process, a NADP+ molecule is also released. (1)

Reducing agents are important in anabolism. The formation of many of the molecules your cells need (for example, all fatty acids) relies on NADPH as a reducing agent to help drive the needed oxidation-reduction reactions forward.

The chemical reaction on the left is of the reaction that creates a molecule of the fatty acid palmitate. As you can see, before the reaction there are 14 NADPH molecules and afterwards 14 NADP+ molecules. (2)

Recycling ADP and NADP+

Just as ADP molecules can (with the help of energy and a phosphate group) be turned into new ADT molecules, NADP+ molecules can (also with the help of energy and a hydride anion) be turned back into NADPH molecules. And so - at least at a cellular level - you recycle.