The Inorganic Chemistry of Vitamin B12

The very first “vegan” question I ever got was the, “how do you get enough B12?” I thought it would be the “protein” or “what do you eat” question, but the B12 was a shocker. I knew it was possible, but I didn’t expect it. I take a multi-vitamin each day, and I eat foods fortified with B12. I’m OK in that department.

Why do vegans usually need B12 supplementation? Well, in general, humans historically got B12 from meat and animal products or from root vegetables. Certain microbes and what not react to naturally enrich these vegetables with B12. The B12 could also enrich plant life through fertilization with animal waste, in the way that ecosystems naturally recycle resources. When chemistry developed artificial fertilizers, herbicides, and pesticides that resulted in the modern agricultural revolution and population explosion, a byproduct was an unnatural inhibition of the pathway nature used to provide B12 to humans through plants. One hundred years ago, vegans likely had no problem obtaining plenty of B12. Today, it’s nearly impossible without outside supplementation. Too little (and too much) B12 leads to potentially fatal health problems. But, what is vitamin B12 and how does it actually work in our bodies? The Vegan Chemist is here to explain it to you as simply as I can.

Vitamin B12 is a relatively simple molecule with a metal at its center. The metal in B12 is cobalt, Co. The Co is bound to an organic molecule (meaning, carbon-based) known as a ligand, and this ligand attaches to the Co through six “donor” atoms to produce six cobalt-atom bonds to the organic ligand. These six atoms arrange around the Co in what is referred to as an octahedral geometry. This is the shape you obtain if you take two pyramids and place them together base to base, where Co would be at the center of this diamond-like structure. Here, two of the atoms arrange themselves at the top and bottom of the Co, like the north and south poles are to the Earth. We call these the axial atoms and bonds, and these are able to be pushed aside and replaced with other molecules as needed. It is this action that allows B12 to attach to other molecules in our body to provide its critical biological function. The remaining four atoms arrange themselves in an “X” formation around the equator of our metal, hence we refer to these as the equatorial positions. Two of the atoms point toward you from the equator of the metal, while two face away from you, behind the metal. These atoms bond very strongly to the metal, anchoring it in place within the scope of the overall molecule. Without the strong bonding these four atoms provide in B12, the Co metal would easily release itself from the ligand, acting as a “free” metal ion. While B12 at proper concentrations is essential to human life, free cobalt ions are dangerous.

Our bodies use vitamin B12 to help activate certain biochemical reactions necessary for life that would otherwise be too slow or too ineffective to occur by themselves. These kinds of molecules are known as catalysts. In our bodies, we have biomolecules that are largely or entirely organic-based in nature that also serve as catalysts. Such biomolecules are called enzymes. Sometimes these enzymes need a partner to help jump-start the reaction(s) that they are designed to facilitate. Vitamin B12 acts in this role, serving as a cocatalyst for certain enzymes in our body. Without it, these enzymes cannot function, resulting in disease and eventual death. The key to B12’s activity is its metal center.

Cobalt is a transition metal. Our bodies require various amounts of different transition metals (e.g., iron, copper, etc.), each playing crucial roles through a wide range of biological functions. One function is to catalyze chemical reactions within our bodies. Without these metals, many biochemical reactions necessary for life would occur too slowly, in too low of a quantity, or not at all. This ability to transfer electrons also means that sometimes metals can be used as a way to easily get rid of an unwanted electron, or sometimes provide an electron necessary for a reaction to proceed. Co acts in this latter way in B12; it can give upwards of two electrons to assist in crucial biochemical transformations.

There are several features that make certain metals ideal candidates to promote chemical catalysis: electronic structure, geometric structure, chemical characteristics, and general reactivity. The role of metal geometry and basic metal-ligand interactions were already discussed above in the B12 system. The unique atomic arrangement of protons, neutrons, and electrons results in the fundamental electronic and chemical nature of each atom. Specific reactions in our body have been tuned specifically to the chemistry of Co, and the organic ligand attached to Co allows for the transport, specificity (i.e., “targeting” and/or binding ability), and safe use of Co within our bodies.

Chemically speaking, Co is known as a hard acid. Acids react with bases, which is why acid rain, the ocean, etc. are corrosive. As an acid, Co has a high positive charge and wants to satiate that charge with a base that has a full or partial negative charge. You know, opposites attract – like charges repel one another, but opposite charges attract. Well, at least in terms of electrical charge that’s the adage. In terms of Co’s inherent reactivity, its chemical “hardness”, the rule is reverse. Here, hard acids prefer to react with hard bases, and soft acids with soft bases. Opposites don’t attract in this case. Together, these features describe the basic function of Co in B12.

The exact “hardness” of Co coupled with its acidity and precise ligand design is how nature has fine-tuned B12 for specific reactions in our body. This is one way that biology evolved to use chemistry with such high precision. Certain enzymes in the body require the electrons that Co can provide, and nature designed these enzymes to the unique chemistry of Co as it exists within vitamin B12. When it finds vitamin B12 floating in the body, it latches onto the Co and nabs an electron so that it can do the job that it needs to do.

Thank you for following my posts, and I hope you enjoy learning about how chemistry relates to a vegan lifestyle. I will have another post in a week or two about the chemistry of veganism!

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