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Dec 1, 2016 · Radical rebound mechanism: a reaction manifold for versatile biotransformations other than oxygenation. The stepwise events of hydrogen abstraction and radical recombination in the rebound mechanism represents a general strategy exploited by nature to perform controllable radical reactions.
- Xiongyi Huang, John T. Groves
- 10.1007/s00775-016-1414-3
- 2017
- J Biol Inorg Chem. 2017; 22(2): 185-207.
- Overview
- Introduction
- Overview of fuel breakdown pathways
- Electron carriers
- Redox reactions: What are they?
- What about gaining and losing H and O atoms?
- What's the point of all this redox?
Intro to redox in cellular respiration. Substrate-level vs. oxidative phosphorylation. Electron carriers.
Let’s imagine that you are a cell. You’ve just been given a big, juicy glucose molecule, and you’d like to convert some of the energy in this glucose molecule into a more usable form, one that you can use to power your metabolic reactions. How can you go about this? What’s the best way for you to squeeze as much energy as possible out of that glucose molecule, and to capture this energy in a handy form?
Fortunately for us, our cells – and those of other living organisms – are excellent at harvesting energy from glucose and other organic molecules, such as fats and amino acids. Here, we’ll get a high-level overview of how cells break down fuels. Then, we'll take a closer look at some of the electron transfer reactions (redox reactions) that are key to this process.
The reactions that extract energy from molecules like glucose are called catabolic reactions. That means they involve breaking a larger molecule into smaller pieces. For example, when glucose is broken down in the presence of oxygen, it’s converted into six carbon dioxide molecules and six water molecules. The overall reaction for this process can be written as:
C6H12O6 + 6O2 → 6CO2 + 6H2O ΔG=−686kcal/mol
In a cell, this overall reaction is broken down into many smaller steps. Energy contained in the bonds of glucose is released in small bursts, and some of it is captured in the form of adenosine triphosphate (ATP), a small molecule that powers reactions in the cell. Much of the energy from glucose is dissipated as heat, but enough is captured to keep the metabolism of the cell running.
As a glucose molecule is gradually broken down, some of the breakdowns steps release energy that is captured directly as ATP. In these steps, a phosphate group is transferred from a pathway intermediate straight to ADP, a process known as substrate-level phosphorylation.
Many more steps, however, produce ATP in an indirect way. In these steps, electrons from glucose are transferred to small molecules known as electron carriers. The electron carriers take the electrons to a group of proteins in the inner membrane of the mitochondrion, called the electron transport chain. As electrons move through the electron transport chain, they go from a higher to a lower energy level and are ultimately passed to oxygen (forming water).
As an electron passes through the electron transport chain, the energy it releases is used to pump protons (H+ ) out of the matrix of the mitochondrion, forming an electrochemical gradient. When the H+ flow back down their gradient, they pass through an enzyme called ATP synthase, driving synthesis of ATP. This process is known as oxidative phosphorylation. The diagram below shows examples of oxidative and substrate-level phosphorylation.
Electron carriers, also called electron shuttles, are small organic molecules that play key roles in cellular respiration. Their name is a good description of their job: they pick up electrons from one molecule and drop them off with another. You can see an electron carrier shuttling electrons from the glucose breakdown reactions to the electron transport chain in the diagram above.
There are two types of electron carriers that are particularly important in cellular respiration: NAD+ (nicotinamide adenine dinucleotide, shown below) and FAD (flavin adenine dinucleotide).
When NAD+ and FAD pick up electrons, they also gain one or more hydrogen atoms, switching to a slightly different form:
NAD+ + 2e− + 2H+ → NAD H + H+
FAD + 2e− + 2 H+ → FADH2
And when they drop electrons off, they go neatly back to their original form:
Cellular respiration involves many reactions in which electrons are passed from one molecule to another. Reactions involving electron transfers are known as oxidation-reduction reactions (or redox reactions).
You may have learned in chemistry that a redox reaction is when one molecule loses electrons and is oxidized, while another molecule gains electrons (the ones lost by the first molecule) and is reduced. Handy mnemonic: “LEO goes GER”: Lose Electrons, Oxidized; Gain Electrons, Reduced.
The formation of magnesium chloride is one example of a redox reaction that nicely matches our definition above:
Mg+Cl2→Mg2++2Cl−
In this reaction, the magnesium atom loses two electrons, so it is oxidized. These two electrons are accepted by chlorine, which is reduced.
However, as Sal points out in his video on oxidation and reduction in biology, we should really put quotes around "gains electrons" and "loses electrons" in our description of what happens to molecules in a redox reaction. That's because we can also have a reaction in which one molecule hogs electrons rather than fully gaining them or is hogged from rather than fully losing them.
Oxidation and reduction reactions are fundamentally about the transfer and/or hogging of electrons. However, in the context of biology, there is a little trick we can often use to figure out where the electrons are going. This trick lets us use the gain or loss of H and O atoms as a proxy for the transfer of electrons.
In general:
•If a carbon-containing molecule gains H atoms or loses O atoms during a reaction, it’s likely been reduced (gained electrons or electron density)
•On the other hand, if a carbon-containing molecule loses H atoms or gains O atoms, it’s probably been oxidized (lost electrons or electron density)
For example, let’s go back to the reaction for glucose breakdown:
C6H12O6 + 6O2 → 6CO2 + 6H2O
Now that we have a better sense of what a redox reaction is, let's spend a moment thinking about the why. Why does a cell go to the trouble of ripping electrons off of glucose, transferring them to electron carriers, and passing them through an electron transport chain in a long series of redox reactions?
The basic answer is: to get energy out of that glucose molecule! Here is the glucose breakdown reaction we saw at the beginning of the article:
C6H12O6 + 6O2 → 6CO2 + 6H2O ΔG=−686kcal/mol
Which we can rewrite a bit more clearly as:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy!
As Sal explains in his video on redox reactions in respiration, electrons are at a higher energy level when they are associated with less electronegative atoms (such as C or H ) and at a lower energy level when they are associated with a more electronegative atom (such as O ). So, in a reaction like the breakdown of glucose above, energy is released because the electrons are moving to a lower-energy, more "comfortable" state as they travel from glucose to oxygen.
Jul 12, 2023 · What happens if you add the hydrogen to the carbon atom at the right-hand end of the double bond, and the chlorine to the left-hand end? You would still have the same product. The chlorine would be on a carbon atom next to the end of the chain - you would simply have drawn the molecule flipped over in space.
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Apr 19, 2023 · If a Br~nsted base combines with hydrogen ions, it shifts the equilibrium of equation 5-5 in favor of dissociation until balance is restored. More hy~ droxide ions are formed in the process, so in water a Br~nsted base is an Arrhenius base as well. In aqueous solution, acids are classified as either strong or weak.
To hydrogenate an alkene, you need hydrogen gas and a metal catalyst, something like platinum or palladium or nickel. And there are many others, but these are the ones most commonly used. So what happens is those two hydrogens from the hydrogen gas are added across their double bond.
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Dec 1, 2016 · The stepwise events of hydrogen abstraction and radical recombination in the rebound mechanism represents a general strategy exploited by nature to perform controllable radical reactions. As discussed in the previous section, the behavior of the incipient substrate radical is highly tunable.
Oct 2, 2017 · For more than 40 years, chemists and biologists have used the radical rebound mechanism to describe what happens when enzymes with transition metals, such as cytochrome P450, hydroxylate C–H bonds. Such enzymes first remove a hydrogen atom from a C–H bond using a high-valent metal-oxo species.
