The trapping of non-metals by biological systems must also depend on the use of membrane restrictions, thermodynamic binding traps, and kinetically energized binding traps and steps. However, unlike ions, neutral molecules Table 2.16 Pumped gradients of metals and their complexes
Metal Inward to cytoplasm Outward to Organelle or
vesicle Fe3+ Ferroxamines, etc. (Ferritins?)
Transferrin Citrate Mitochondria Co2+ Vitamin B
12 ?
Ca2+ (Free ion) Free ion Reticula,
outside
Na+ (Free ion) Free ion Outside
Mg2+ Free ion (Free ion) (Outside)
H+ Free ion and many ligands Free ion or with Vesicles, outside
many ligands K+ Free ion (Free ion)
Cd2+ ? Metallothionein Outside
HPO2–
4 Free ion Several organic Mitochondria
phosphates
Cl– Free ion Free ion Outside
Cu2+ Albumin Metallothionein Outside
such as N2, O2, H2O, CO2, NH3, Si(OH)4, and B(OH)3carry N, O, H, C, Si, and B to all parts of a cell and cannot be prevented from free diffusion across membranes. Of these, compounds such as B(OH)3are readily trapped by binding to cis-diols (see margin), probably to specific polysaccharides, giving moderately labile condensation products.
Apparently similar reactions can be written for Si(OH)4 or any weak dibasic acid and we find these structures for phosphorus in cyclic adenosine phosphate. We also note the occurrence of boron in the natural compound boromycin and it could well be that silicon in mucopolysaccharides is another example of this type of binding. In this context an element such as molybdenum, visualized as Mo(OH)6, is similar to Si(OH)4(Chapter 17). The ring structures decrease in thermodynamic stability, in the order B > Si > P, so that, for the most part, the later non-metal molecular hydroxides are retained by hydrogen bonding rather than covalent complexing, at least initially.
The other molecules, H2, N2, O2, H2O, CO2and NH3, are less reactive and some need to be activated, reduced, or oxidized, before they can be bound. Their uptake is controlled by use of energy and kinetics of reactions and not by thermodynamics (see Fig. 1.8). After the initial capture, they usually pass into small molecule intermediates, such as charged amino acids and sugars, which remain trapped in cells by the membranes. These intermediates are
covalent (as opposed to ionic) kinetic traps and, as small charged molecules,
can be retained behind membranes before further synthesis leads to polymer formation.
Among these small non-metal molecules, dioxygen and dinitrogen are first absorbed at metal centres. Organic molecules can hardly activate them. For O2 several types of centre are known, such as haemoglobin (Fe(II)), haemerythrin (Fe(II) . . . Fe(II)), and haemocyanin (Cu(I) . . . Cu(I)). In the last two cases oxygen is held as O2
2–while in haemoglobin it binds reversibly as O2, although there is partial charge transfer from iron to give a formal O•
2 –
state. If oxygen is to be incorporated, the final capture of the element involves the attack on a substrate by O2activated at a metallo-enzyme centre similar to the dioxygen carrier centres to give oxygen containing organic molecules. In some cases O2 is reduced to superoxide and then to peroxide, which can also be utilized in various reactions or taken down to H2O, the last stage of reduction. The uptake of N2is known to be by the iron-molybdenum enzyme nitrogenase which car- ries out the reaction
The reduction is very energy-expensive in that it uses many ATP molecules. It is effectively irreversible through this use of energy, but also because the resulting ammonia goes on to be incorporated in organic molecules by con- densation reactions. The question why the more active O2is not taken up by the N2-absorbing sites is puzzling and still unsolved. Perhaps O2is removed first and its access to the site is prevented, or N2is indeed picked up by a spe- cial centre with little or no affinity at all for dioxygen, but this is not very com- mon in chemistry, except for compounds involving metals that are not used in biology, e.g. ruthenium.
As to CO2, it is rapidly converted to bicarbonate by the zinc-enzyme car- bonic anhydrase or reacts directly with an activated organic molecule. The incorporation is again energy-driven by ATP. In this case there is no interfer-
N2 H O + 6e2 NH OH many ATP molecules 3 – +6 æ ææ Ææ 2 + 6
ence by O2or N2, since CO2is a quite different molecule with a quadrupole structure that allows a different type of binding. After incorporation, the CO2 is reduced to states equivalent to HCHO, –CH2–, etc. in organic compounds. Incorporation of -CH3from CH4involves typical organometallic chemistry (see Chapter 16).
Some of the above and all the other non-metals are also available to bio- logical systems as simple anions, e.g. F–, Cl–, Br–, I–, or oxyanions such as CO2–
3, HCO–3, H3SiO–4, NO–3, H2PO–4, HPO2–4, B(OH)–4, SO2–4 , and SeO2–4. Molybdenum and vanadium can also be included in this group as MoO2–
4 and
VO3–
4 and the principles of uptake for all these elements are similar. Naturally, the ions cannot cross membranes without carriers, and, once captured, they can be retained by such physical separation devices. The problem of capturing these non-metal ions by biological systems is therefore not very different from that of the capture of metals. We shall need to look for anion pumps, exchang- ers, and channels and their selectivity principles, as well as at simple thermo- dynamic control over incorporation (see Fig. 2.15). Once inside a cell several of them can be reduced and differential non-metal incorporation is then great- ly assisted. (Note that apart from essential uptake cells must reject poisonous anions, e.g. HAsO2–
4 and here the selective direction, in or out, of pumps is a critical factor).