IMPORTANT REACTIONS IN METABOLISM:

MICROORGANISMS AND BEYOND

FADH 2 ½ O 2 FAD H O 2

4.12 IMPORTANT REACTIONS IN METABOLISM:

OxIDATION AND HYDROLYSIS

Enzymes catalyze a wide range of chemical reactions. Two very important reactions are found throughout metabolite pathways and are crucial to truly understand some interesting changes that occur when cooking and baking. One set of reactions involves the transfer of electrons and hydrogens (oxidation/reduction), and the other uses water to break a bond or produces water to make a bond (hydrolysis and dehydration).

These are only two very common chemical changes we see in food and cooking. Let’s start with oxidation and reduction.

4.12.1 Oxidation and Reduction

As we learned earlier, ATP is an important product of metabolism and can be pro-duced through reactions catalyzed by glycolysis. In addition to the direct phosphor-ylation of ADP seen in glycolysis, ATP can also be generated indirectly through an oxidation reaction. What is an oxidation reaction? Historically, if an element reacted with oxygen to produce an oxide, you would classify the reaction as an oxidation reaction. You know this type of reaction well, as it describes the conversion of ele-mental iron (Fe) to iron oxide (Fe₂O₃), also known as rust:

2 Fe s( ) 3 O g₂( ) 2 Fe O s_{2 3}( )

Now, an oxidation reaction has a much broader meaning. Oxidation reactions occur when an element, compound, or ion loses electrons (Fig. 4.26).

How do you know when an oxidation reaction takes place? You assign an oxidation number to each atom in a compound, which indicates whether the atom is electron rich, neutral, or electron poor. If the oxidation number for a particular atom changes over the course of a reaction, you have an oxidation reaction.

Oxidation reactions occur all around us and are key in food preparation and cooking processes. For example, some of the color differences that you see in meat are due to numerous oxidation reactions that occur in the iron (Fe) that is found in the muscle protein, myoglobin. As the iron changes its oxidation state (between +2 and +3) and its bonding partner (O₂ or H₂O), it changes color, resulting in a color change in the meat (Fig. 4.27).

Oxidation

Atom loses an electron Atom gains an electron Reduction

FIGURE 4.26 Oxidation and reduction. 

Schematic of iron (Fe) bound to heme in myoglobin

Myoglobin heme has changed to iron 3⁺ and can no longer bind oxygen = brown Myoglobin heme binding water = purple

(this heme has just donated oxygen) Myoglobin heme binding oxygen = red

(this myoglobin is ready to donate oxygen)

The conversion of purple to brown occurs when there is a lack of

oxygen for an extended time The heme iron cannot be

“empty.” If the heme has just donated oxygen to an enzyme, then water will take its place

Enzymes in raw meat can convert brown back to purple CH₂

CH₂

CH₂CH₂CO₂^–

O₂ OH₂

Fe²⁺ Fe²⁺

CH₂CH₂CO₂^– CH₃ CH₃

CH HC

C C

C

C N

N N

N Fe²⁺

C C

C

N

N

N

N N

N

OH₂ Fe³⁺

N

N N

N N N

C C

C C C

HC

HC

HC C

C C C H₃C

H₃C HC

FIGURE 4.27 Myoglobin and iron. The oxidation state of iron held in place by the heme of myoglobin is shown here.

In raw fresh meat, iron has an oxidation state of +2; this means that the iron has two fewer electrons than protons, since protons are positively charged and electrons are negatively charged. Iron in an oxidation state of +2 in myoglobin typically has a red or pink color associated with it. As meat cooks, the iron is oxidized and loses one electron, so its oxidation state increases by one (there are now three more posi-tively charged protons than negaposi-tively charged electrons). In other words, the iron is oxidized to Fe³⁺. Iron in myoglobin with a +3 oxidation state is typically brownish in color, as is observed in well‐done beef.

A different type of oxidation reaction occurs when you watch a cut apple turn brown. A browning apple actually undergoes two different oxidation reactions. In the first reaction, the oxidation reaction is simply identifiable by the addition of oxygen to a molecule, similar to what you see in the elemental iron to rust reaction. Oxygen is added to the monophenol and electrons are lost (which is not obvious in this example) to yield catechol.

In the second oxidation reaction (catechol to o‐quinone), no additional oxygen is added, but the catechol undergoes a dehydrogenation reaction; it loses hydrogens.

This is another way to identify and define oxidation reactions, particularly those in which oxidation numbers are not obvious. Following o‐quinone production, a variety of enzyme‐ and nonenzyme‐catalyzed reactions take place to yield colored com-pounds like melanin, which is the color that we observe in our browning apple.

Have you ever noticed that a partially used bottle of wine eventually develops a sour taste? Upon exposure to air (for a long time), the ethanol in the wine oxidizes to acetic acid (Fig. 4.28). This process doesn’t happen very readily on its own, but occurs in the presence of enzymes that are produced by a bacterium called Acetobacter aceti. When vinegar is made, this type of bacteria is purposefully added to wine or other fermented alcoholic beverages to produce the vinegar that you might use in making salad dressings or dyeing Easter eggs.

Using the myoglobin, apple browning, and vinegar reactions as examples, you now have the expertise to identify oxidation reactions. If there is a loss of electrons (myoglobin), a gain in oxygen (browning and vinegar), or a loss of hydrogen (brown-ing and vinegar), you have an oxidation reaction. To be clear, in all of these reactions, a compound, element, or ion loses electrons. In the case of the browning apple and vinegar, it is harder for a novice to identify the electron loss.

Where do the electrons go? In these examples, it appears that the electrons just get lost to some unknown place in the surroundings. However, this is not the case. When oxidation reactions occur, the electrons lost by one molecule must be transferred to or gained by another molecule. The molecule that gains the electrons is reduced. If a molecule loses electrons, another molecule must be available to accept the electrons.

OH OH

O

Ethanol Acetic acid

Vinegar

FIGURE 4.28 Ethanol conversion to acetic acid. 

In other words, whenever an oxidation occurs, a reduction must also occur. If you think about oxidation/reduction reactions in terms of hydrogen and oxygen, the mol-ecule that is oxidized gains oxygen or loses hydrogen, while the reduced molmol-ecule gains hydrogen or loses oxygen (Fig. 4.26).

Within a cell or microorganism, the ultimate acceptor of the electrons lost in an oxidation reaction is oxygen. In the process, the oxygen also gains two hydrogen ions and is converted to H₂O. However, oxygen does not directly and immediately accept the electrons in every cellular oxidation reaction that occurs. Rather, cells and micro-organisms use an “intermediate” electron acceptor, called a coenzyme, that functions as a temporary electron carrier. A common coenzyme involved in metabolic processes is nicotinamide adenine dinucleotide (NAD⁺).

NAD⁺ serves as an electron carrier by accepting two electrons and two hydrogen cations (protons) from the molecule that is being oxidized, thereby generating NADH plus a proton. In the example shown, the l‐malate is oxidized, while the NAD⁺ is reduced to NADH (Fig. 4.29). NAD⁺ is largely derived from niacin, a B vitamin that is essential for the survival of cells and organisms. Good sources of niacin include animal products (i.e., meat, eggs, seeds, and legumes). However, only small amounts of the NAD⁺ are present in a cell. In order for metabolic processes and oxidation reactions to occur within a cell or microorganism, recycling of NADH to NAD⁺ is a key part of metabolism and how microorganisms meet their energy needs. If you enjoy eating a crusty piece of bread with cheese or drinking a beer, you have benefited from the recycling of NADH to NAD⁺ by yeast and bacteria.

REFERENCE

[1] Volker, M. (2008) Bacterial Fermentation. Goethe Universität, Frankfurt/Main.

Ribo ADP ADP

Ribo

H

NAD⁺ + H⁺ + 2e^– NADH

O O

Oxidation Reduction

NH₂ N

H H N

NH₂

+

FIGURE 4.29 Redox of NAD⁺/NADH. 

The Science of Cooking: Understanding the Biology and Chemistry Behind Food and Cooking, First Edition. Joseph J. Provost, Keri L. Colabroy, Brenda S. Kelly, and Mark A.Wallert.

© 2016 John Wiley & Sons, Inc. Published 2016 by John Wiley & Sons, Inc.

Companion website: www.wiley.com/go/provost/science_of_cooking

5