Design of metabolic and enzymic machinery to fit lifestyle and environment

Biochemical theory, spectacular as its growth has been in the last half-century, finds its experimental basis in a remarkably few organisms. The rat, the ox, the pig, the hen plus a few micro-organisms consitute the primary source of biochemistry's experimental material, while, by contrast, modern systematics estimates a bewildering and often overlooked diversity of over 10(6) animal species. As a result of this highly limited 'sampling' of nature, classical biochemistry has supplied an equally limited insight into how far biochemical mechanisms can be extended or adapted for (1) allowing an organism's exploitation of its environment and (2) accommodating environmental change. The situation has improved in more recent years, so that it is now possible to decipher at least the broad strategic outlines of biochemical adaptation to the environment. This essay focuses particular attention on the influence of O2 availability on muscle metabolic organization and control. By drawing examples of muscle metabolism spanning a range from obligate-anaerobic to obligate-aerobic organization, it is possible to demonstrate that certain kinds of metabolic features are highly conserved, whereas others are changed time and time again, in different combinations and permutations. The latter properties constitute the 'raw material' for adapting metabolism to specific O2 regimes, and include (1) the kinds and amounts of enzymes present in muscle, (2) the fine regulatory circuitry allowing for controlled transition from low to high work loads and (3) compensation mechanisms for the adjustment of enzyme reactions with respect to such external parameters as temperature, pressure, salinity and so forth. Within the inescapable limitations of phylogenetic origins and the time available for responding to specific environmental conditions, adequate adaptations of the fundamental design of muscle metabolism have evolved to exploit environments differing greatly in O2 availability and to accommodate a wide range of muscle functions.