2-10 Small molecules are also important in the cell

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• NAD and FAD are two common proton and electron carriers in the cell.
• Tetrahydrofolate, cobalamin and coenzyme A are common carbon carriers in the cell.
• Many enzymes require minerals for proper function in the cell. Common examples include, iron, zinc, magnesium, and calcium.

There are also a number of important small molecules that shuttle protons, electrons or small carbon moieties around the cell. These small entities typically do their job in association with proteins to which they can be either loosely or tightly bound. All of life on this planet seems to have settled on a surprisingly small set of molecules to perform these tasks. Almost certainly this is because the use of these molecules evolved early and has been maintained through evolution.

Proton and electron carriers

Most amino acids are not particularly good at either donating or accepting electrons and when they do, it is under a limited range of conditions. As you will read in the chapter on metabolism, the ability to move electrons among proteins is critical to all life, so two general types of prosthetic group associated with proteins have evolved for this purpose. Figure 2-19 shows the first type, which are organic multi-ring structures and the other type are iron-sulfur clusters. In both cases, these carriers have characteristic affinities for accepting and donating electrons and protons, but these affinities are also affected by the proteins in which they are found. Thus, a wide range of electron carriers with different properties has evolved. By organizing these carriers in precise patterns in the cell, the cell is able to use the transfer of electrons to do work.

Carbon carriers

There are also small molecules in the cell that serve as carriers of important carbon compounds. Essentially, these carriers have the right chemical properties that make it relatively easy for enzymes to add or remove a particular carbon unit. Tetrahydrofolate and cobalamin (vitamin B₁₂) are often involved in adding or removing one-carbon units during the synthesis of various structures in the cell. Coenzyme A is necessary for the transfer of small 2 to 4 carbon units (acetyl, propyl) from one enzyme to another. It finds utility in both the synthesis and breakdown of organic molecules. The beauty of using a small set of carriers is that it allows the easy movement of carbon from one pathway to another.

Important minerals

Many types of minerals are important for the proper functioning of enzymes. For example, magnesium ions are essential for ATP-binding by many enzymes. Zinc is important in the proper folding of some enzymes and iron, in the form of iron-sulfur centers and hemes, is critical in many electron transport proteins. Minerals also help bind structures in the cell together. For example, magnesium and calcium are necessary for the stabilization of membranes. Potassium ions in the cell shield the large amount of negative charge on the DNA allowing it to pack more tightly together. More will be said in later chapters about their specific roles, but some of the more important ions include K⁺, PO₄^-3, Mg⁺², Zn⁺², Ca⁺², Mn⁺², Fe⁺² and Fe⁺³.

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