THE FUNCTION OF BIOCHEMICAL FUNCTIONS

The other morning, while having breakfast, I took my usual tablet of vitamin B12. I reread the label: ‘Vitamin B12—Reduces tiredness and fatigue’. Looking up the functions of vitamin B12, one might find that this vitamin has a number of functions, such as contributing to the production of red blood cells or mechanisms of DNA renewal, which may reduce tiredness and fatigue. But what does it mean for something like a vitamin, a biochemical macromolecule, to have a function?

Understanding the functions of molecules matters for reasons beyond vitamin supplements. Biochemical functions are discussed in medicine and physiology when, for instance, studying the production of red blood cells, or when exploring how proteins fold and get their functional properties. Moreover, the relation between structure and function is one of the key problems in biochemistry.

So what is a biochemical function? In my article, I explore whether we can answer this question by considering accounts of biological and chemical functions. This analysis allows us to see that neither of these two approaches are satisfactory on their own; instead, it is their synergy that captures the nature of biochemical functions. Both structure and evolution matter when characterizing biochemical functions, since biochemical functions are those chemical properties that contribute to evolved biological processes. But let me proceed in order to illustrate what biochemical functions are.

Much of the discussion on functions comes from reflecting on their role in biology (Garson [2019]). So, some have suggested that the approach taken there could help define biochemical functions too (Bartol [2016]). Functions in biology are usually attributed to traits, such as limbs or organs. For instance, it is commonly said, ‘the function of the heart is to pump blood’. Moreover, biological functions are commonly understood in terms of evolutionary theory and the evolutionary advantage associated with a given trait. These theories are both backwards-looking and forward-looking, and they share the ascription of functions to traits. Backward-looking theories of function state that a trait’s function is a difference-maker effect that contributed, in the past, to ancestral fitness and has been retained via natural selection because of these benefits (Neander [1991]; Mitchell [1993]). Forward-looking theories of function instead define a function as a difference-maker effect of a trait whose manifestation can enhance the organism’s fitness in the future (Bigelow and Pargetter [1987]).

However, vitamins—and other biochemical molecules—are macromolecules that we sometimes do not produce but rather introduce via nutrition. For instance, B12 vitamins are bio-synthesized in nature only by prokaryotic single-celled organisms, such as some bacteria and archaea. Thus, B12 vitamins are not traits or organs of an organism that could (in the past) or can (in the future) enhance our reproductive fitness. Furthermore, it seems that B12 vitamins would still make a difference and have a function, even if they do not enhance the organism’s fitness—for example, if the organism consuming the vitamin has structural anomalies that prevented uptake, or if the organism experiences a lack or an excess of vitamin B12, or there are environmental changes that impede absorption. Consequently, while we are inclined to think that vitamin B12 has a function (as scientific practice considers it so), neither of the biological accounts of function mentioned allows us to say as much. When considering biochemical molecules, functions do not seem to be identified in terms of an evolutionary contribution to fitness.

Another option is to consider a chemical approach to functions, since, after all, these are biochemical molecules. Chemical functions are related to functional classifications of compounds in terms of functional groups, namely, groups of atoms or bonds in a molecule responsible for typical chemical reactions. Chemical functions, so interpreted, are then contributions to chemical reactions. This is in line with what philosophers call a causal theory of function, whereby a function is what something does or is capable of doing within a in a complex system (Cummins [1975]; Wouters [2003]). According to this theory, chemical functions are properties that allow the compounds to contribute to given reactions. As mentioned, the function of vitamin B12 is that of contributing to the process of blood renewal (haematopoiesis). Specifically, this happens because vitamin B12 acts as a coenzyme in the reaction involved in regenerating methionine, required for the normal production of red blood cells. In this sense, when we say that the B12 vitamin has a hematopoietic function in humans, we are referring to its property of reacting in a specific way during the regeneration of methionine in the production of red blood cells. This is close to the chemical view of function. We maybe have found a functional framework for biochemical functions and their ascription.

However, we might not be completely satisfied with this approach, since, after all, biochemical kinds seem to have a dual nature, chemical and biological, and this seems like it ought to be reflected in the nature of biochemical functions as well (see Bartol [2016]; Tahko [2020]). When I read about the function of vitamin B12, this does not refer to a particular chemical structure and related specific atoms and bonds. Rather, it refers to a role within a given biological process or system. The difference-making role that vitamin B12 plays is not as a chemical reaction alone, but as a chemical reaction that is part of a biological process. The biological environment provides a specific context for biochemical kinds, and this suggests that biochemical functions are something more than chemical functions.

What is the solution, then? Should we conclude that biochemical molecules do not have genuine functions? That would not only be counterintuitive, but it goes against the ascription of such functions in multiple scientific and everyday contexts. In ‘Biological Functions’, I argue that a satisfying answer can be found by combining the two approaches. On the one hand, biochemical functions correspond to chemical reactions; on the other, these chemical reactions are so because of the evolutionary history of the traits or processes to which these molecules contribute. Granted this, biochemical functions can be defined as those chemical properties that contribute to the organism’s biological processes. Both structure and evolution matter: structure provides the chemical properties that are associated with biochemical functions; evolution provides the context of action and the identification of the relevant chemical properties. In the case of vitamin B12 and its functions, we can see how its role in reducing fatigue is related to its role in crucial biological processes, such as the renovation of blood cells. This function is an evolutionarily selected chemical contribution that behaves as such because of the evolutionary history of our biological processes.

How we think about the nature of biochemical functions affects how we think about their contributions, and how we explore the beautiful interactions between the chemical and the biological. And this, in turn, raises fascinating questions about how to relate the structural and functional properties of biochemical molecules, further stimulating both scientific and philosophical discussions.