BIOINORGANIC CHEMISTRY A Short Course Second Edition

ATP↔++ADP PO 43 − energy (5.1)

Other important biological manifestations of phosphorus include the pyro-

phosphate anion, PO 274 − or PP i , and inositol 1,4,5 - triphosphate, IP 3. Pyrophos-

phate anions can arise through the following reaction:

(^) ATP→+AMP P O 274 − (5.2)
Note that equation 5.2 is irreversible and the product AMP will require two
phosphorylation steps to reconstitute the high - energy adenosine triphosphate,
ATP. Inositol 1,4,5 - triphosphate is an important molecule in the cytosol, where it
releases calcium ions from storage. It forms part of a series of inositol - phosphate
species that mediate calcium ion concentrations inside and outside the cell.

5.2.2 Potassium, Sodium, and Chloride Ions,

It is diffi cult to separate the discussion of potassium, sodium, and chloride ions

in a physiological context, because the activity of these ions is intimately inter-

connected. These ions maintain osmotic pressure between the inside and

outside of cells, and their passage across cell membranes (as well as that of

the proton) comprises a huge area for discussion. First it is important to know

something about passive diffusion and active transport across cell membranes

and to understand something of the complex chemistry that takes place within

the cell in various vesicles. In general, physiological ionic strength is dependent

on sodium chloride (NaCl) concentration outside the cell and on potassium

chloride (KCl) concentration inside the cell. A deviation of less than 1% in

the concentration of these ions in their proper locations leads to weakness

within any organism. Reference 2 contains much more detailed discussions on

the following topics.

Cell membranes contain selective ion channels that are highly discrimina-

tory for potassium ions, sodium ions, calcium ions, and the proton. For instance,

the highly selective potassium channels of nerves show selectivity for ions as

Li < Na << K > Rb > Cs, and calcium channels show selectivity as Mg << Ca

> Sr > Ba. Ion selection operates on the basis of size and repulsion, not

Figure 5.1 Diphosphoglycerate (DPG), regulator of heme dioxygen (O 2 ) binding.

O-

O

CH

CH 2

P O

O

O-

O-

• O
  P

O

O

• O
  C

HOMEOSTASIS OF METALS (AND SOME NONMETALS) 193