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3'-Phosphoadenylyl sulfate, abbreviated PAPS, is a nucleotide used as an activated donor in sulfation. Its adenosine unit bears phosphate groups at the ribose 3′ and 5′ positions; a sulfo group is attached to the 5′ phosphate. That covalent arrangement distinguishes PAPS from free sulfate dissolved alongside an unrelated nucleotide.^[ref]

A concrete example is heparan sulfate glucosamine N-sulfotransferase. The enzyme transfers the sulfur-containing group from PAPS to a glucosamine nitrogen within heparan sulfate, leaving adenosine 3',5'-bisphosphate. Abbreviating that product as PAP, the transfer can be summarized schematically asThis schematic omits proton and charge bookkeeping; the IUBMB reaction specifies the actual polymer-associated acceptor.^[ref]

The example separates donor availability from product specificity. PAPS supplies a transferable group, while the enzyme determines which acceptor and attachment site participate. Supplying sulfate is therefore not, by itself, a specification of which position in a glycosaminoglycan becomes modified.

Adenosine 3′,5′-bisphosphate, often abbreviated PAP, is a nucleotide with separate phosphate groups attached at the 3′ and 5′ positions of the ribose in adenosine. The positions are part of its identity: two phosphate groups on different sugar positions do not make the same structure as the linked phosphate chain of adenosine diphosphate.^[ref]

An important reaction example is sulfation using PAPS. In the reaction catalyzed by heparan sulfate glucosamine N-sulfotransferase, PAPS transfers a sulfur-containing group to a glucosamine unit in heparan sulfate, leaving adenosine 3',5'-bisphosphate. The product retains the two phosphate groups but lacks the transferred sulfo group. Counting phosphorus atoms alone would therefore fail to distinguish donor from product.^[ref]

This is a useful example of why a molecular formula or a count of functional groups is less informative than a chemical structure: attachment positions and connectivity determine which molecule is present.

A lead-acid battery is suitable. Its positive electrode reduces PbO2 to PbSO4 and its negative electrode oxidizes Pb to PbSO4. The net discharge reaction isUsing the supplied rounded potentials gives $E^{\circ }=1.70-(-0.35)=2.05\mathrm{V}$. The Nernst equation, taking solid and water activities as unity, givesAt $\mathrm{pH}=1$ and $298\mathrm{K}$,Thus $1.93\mathrm{V}$ if sulfate activity is assumed unity. The pH alone does not specify sulfate activity, particularly where bisulfate formation and nonideal concentrated acid matter; without that assumption the numerical voltage is underdetermined.

For recharge, reverse both electrode reactions without irreversible loss of active material. Use porous, electronically connected Pb and PbO2 on acid-compatible conducting grids, with enough pore volume for PbSO4 formation and electrolyte transport. Fine, reversible sulfate deposits are desirable; coarse persistent crystals cause sulfation. The separator must remain acid-resistant, electronically insulating and permeable to electrolyte ions while preventing shorts; selective ion exchange is not intrinsically required. A compatible lead-alloy grid is preferable to assuming any metallic substrate survives the positive electrode's oxidizing acidic environment. Control current and charging voltage to limit hydrogen evolution, oxygen evolution, water loss, corrosion, heating and active-material shedding. Nickel hydroxide-based alkaline electrodes are poor choices in this acidic electrolyte because their phase stability and reaction conditions differ from those of their tabulated alkaline half-reactions.