Question

In a chemical reaction, NADH is converted to \(\mathrm{NAD}^{+}+\mathrm{H}^{+} .\) We say that NADH has been a. reduced. b. phosphorylated. c. oxidized. d. decarboxylated. e. methylated.

Short answer

c. oxidized

Step-by-step solution

Determine what is a reduction

In a reduction, an atom, ion, or molecule gains electrons. It's called a reduction because the total amount of positive charge in the species is reduced.

Determine what is phosphorylation

Phosphorylation is a biochemical process where a phosphate group is added to an organic molecule. In this case, looking at the reaction, no phosphate group has been added to NADH, hence, NADH is not phosphorylated.

Determine what oxidation is

Oxidation in biochemical terms usually means the loss of electrons, sometimes associated with a molecule losing a hydrogen atom, as is the case here with NADH converting into NAD+ and H+.

Determine what decarboxylation is

Decarboxylation is a chemical reaction where a carboxyl group is removed from a molecule. In this reaction, there has been no carboxyl group removed, hence, NADH has not been decarboxylated.

Determine what methylation is

Methylation is a process by which a methyl group (CH3) is added to a molecule. In this reaction, there hasn't been any methyl group added, hence, NADH has not been methylated.

Determine which process applies to the conversion of NADH.

Given that NADH has lost an electron and a hydrogen atom in this reaction, this is an oxidation. Therefore, say that NADH has been oxidized.

Key concepts

Redox Reactions in Biochemistry

Understanding redox reactions—or oxidation-reduction reactions—is fundamental in grasping the complex processes that occur in biological systems. At its core, a redox reaction involves a transfer of electrons between two entities. In biochemistry, these reactions are crucial for energy production, detoxification, and signaling pathways.

In a redox reaction, we see one substance being oxidized by losing electrons, while another is reduced by gaining electrons. It's important to remember the mnemonic “OIL RIG”: Oxidation Is Loss, Reduction Is Gain. These processes often occur concurrently, with the electron donor being oxidized and the electron acceptor being reduced. The movement of electrons from one molecule to another is a form of energy transaction in cells, as the potential energy of electrons can be harnessed for biological work.

Furthermore, in biological systems, redox reactions often involve the transfer of hydrogen atoms. Since hydrogen atoms consist of a proton and an electron, their transfer automatically entails the movement of electrons. Thus, when discussing whether a molecule has been oxidized or reduced, we look for gains or losses of both electrons and hydrogen atoms.

NADH and NAD+ in Cellular Respiration

Molecules such as NAD+ (nicotinamide adenine dinucleotide) and its reduced form NADH play a pivotal role in cellular respiration. These are coenzymes that act as electron carriers within the cell. During cellular respiration, NAD+ is reduced to NADH in various metabolic pathways like glycolysis and the Krebs cycle.

NADH then carries the electrons to the electron transport chain, located in the inner mitochondrial membrane. There, NADH is oxidized back to NAD+, and the electrons it carried are transferred through a series of proteins. This electron transfer releases energy, which is used to create a proton gradient across the mitochondrial membrane, ultimately leading to the production of ATP through a process known as chemiosmosis. Thus, NADH and NAD+ are essential for the efficient transfer of energy from food molecules to ATP, the energy currency of the cell.

Significance of NAD+/NADH in Metabolic Control

The ratio of NAD+ to NADH within a cell is also an indicator of the cell's metabolic state. A high NAD+/NADH ratio typically signals that the cell is in an energy-producing mode, while a low ratio indicates that the cell is consuming energy. By regulating this ratio, the cell can control which metabolic pathways are active at any given time.

Biochemical Electron Transfer

Biochemical electron transfer is integral to many cellular processes, including photosynthesis and cellular respiration. This fundamental mechanism involves the movement of electrons through a series of protein complexes and other molecules.

Two key features characterize these electron transfers: They are often coupled with a change in the energy state of the electron, and they are mediated by specialized molecules, like NADH, FADH2 (flavin adenine dinucleotide), and ubiquinone. These molecules can accept and donate electrons readily, making them perfect for ferrying electrons between different locations within the cell.

Protein Complexes in Electron Transfer

In the electron transport chain, a series of protein complexes and small electron carriers transport electrons, step by step, toward a final electron acceptor, such as oxygen. With each transfer, energy is released, which is then used to pump protons across the mitochondrial membrane, generating the electrochemical gradient necessary for ATP synthesis. The precision and coordination of these transfers are vital, as misplaced electrons can generate harmful reactive oxygen species. The study of electron transfer is crucial to understanding not only how cells produce energy but also how they protect themselves against oxidative damage.