200 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS
html. Na + /K + - ATPase belongs to the P 2 subtype of the P - type ATPase super-
family and is composed of two subunits.^12 The informative database website
http://www.patbase.kvl.dk/ , classifi es Na + /K + - ATPase as a P 2C ATPase. This
class also includes the H + /K + - ATPases that will not be discussed here. The
Na+ /K + - ATPase catalytic alpha subunit ( ∼ 113 kDa) contains 10 transmem-
brane segments, binds ATP as well as both sodium and potassium ions, and
includes the amino acid phosphorylation site. The alpha subunit also contains
three cytoplasmic domains: (1) the A - (anchor or actuator) domain, (2) the
N - domain that binds the ATP nucleotide, and (3) the P - domain that contains
the aspartic acid residue to be phosphorylated. The Na + /K + - ATPase amino acid
residue numbering system used here is that from pig. The domains include the
following: (1) N - domain, residues arg378 – arg589; (2) P - domain, residues
leu354 – asn377 and ala590 – leu773; (3) phosphorylation site, asp369. The smaller
beta subunit (a∼ 55 - kDa glycoprotein) is necessary for activity of the complex.
It appears to be critical in facilitating plasma membrane localization and acti-
vation of the alpha subunit. Potassium and sodium cation transport occurs
during a cycle of conformational changes in the enzyme that involve phos-
phorylation of a specifi c amino acid — asp369 in the P - domain. As currently
understood, the sequence of events can be summarized as follows: (1) The
pump, with bound ATP, binds three intracellular Na + ions; (2) ATP is hydro-
lyzed, leading to phosphorylation of an amino acid residue within the cyto-
plasmic loop of the pump; (3) conformational changes in the pump expose the
Na+ ions to the cell ’ s exterior, where they are released; (4) the pump binds two
extracellular K + ions, perhaps concurrently with dephosphorylation of the
alpha subunit; (5) ATP binds and the pump reorients to release K + ions into
the cytoplasm (the inside of the cell); and (6) the cycle begins again. The details
of the cycle are better known for the Ca 2+ - ATPase system and these are dis-
cussed in Section 6.4.2.
The fi rst X - ray crystallographic structure of the Na + /K + - ATPase N - domain
(at 2.6 - Å resolution) was published in 2003, and the data deposited in the
Protein Data Bank as PDB: 1Q3I.^13 (Note the PDB accession number contains
number one “ 1 ” as its fi rst digit and the letter “ I ” as its last.) This author ’ s
explanation of the enzyme ’ s cycle includes the following details: (1) The A -
domain is formed by a polypeptide segment between transmembrane α - helices
2 and 3 and includes the enzyme ’ s N - terminal end; and (2) a larger loop
(extending into the cell ’ s cytoplasm) between transmembrane α - helices 4 and
5 forms the P - domain, into which the N - domain is inserted. As stated previ-
ously, the N - domain contains the ATP binding site and the P - domain contains
the aspartic acid residue that is phosphorylated (asp369). Subsequently in the
cycle, ADP is released and hydrolysis of the phosphorylated residue takes
place. These events are coupled to the active transport of three sodium ions
(3 Na + ) out of the cell and two potassium ions (2 K + ) into the cell. (See Figure
5.3 .) The transmembrane helices are intimately involved in this transport. The
A - , N - , and P - domains undergo large conformational movements during the
transport cycle, alternating between the so - called E 1 and E 2 states. (See more