3. タンパク質化学実験法
はじめに
まずは、以下の文章をよーく読んでください。
Phosphagen
(guanidino) kinases constitute a family of highly
conserved enzymes which catalyze the reversible transfer
of phosphate
from phosphagen (creatine phosphate (CP), arginine phosphate
(AP),
glycocyamine phosphate (GP), lombricine phosphate (LP),
taurocyamine
phosphate (TP), hypotaurocyamine phosphate (HTP)) to
ADP yielding
ATP. Creatine kinase (CK), the most widely studied member
of this family,
plays a central role in both temporal and spatial ATP
buffering in cells
which display high and variable rates of energy turnover
(Fig.1). The
remaining major members of this family (arginine kinase
(AK),
glycocyamine kinase (GK), lombricine kinase (LK), taurocyamine
kinase
(TK), hypotaurocyamine kinase (HTK)) have not been investigated
as
extensively as CK but likely play a similar physiological
role.
The molecular
properties of CK have been the subject of intensive
study over the past 40 years which recently culminated
in the publication
of a crystal structure for this enzyme. In birds and
mammals CK exists as
three dimeric, cytoplasmic isoenzymes (MM-, MB- and BB-CK)
as well as
two mitochondrial isoforms (Mia, acidic; Mib, basic)
which assemble into
homo-octamers (Mia-CK; Mib-CK) and dimers in vivo. Lower
vertebrates
contain a number of dimeric, cytoplasmic CKs and
echinoderms, which
have close affinities to chordates, have a unique contiguous
trimeric CK
which is present in sperm flagella and likely in somatic
tissues. Furthermore,
Wyss et al. have recently shown that echinoderm
sperm possess an Mi-CK
which exists primarily as octamers. Cladistic analyses
of available
vertebrate and echinoderm CK sequences have shown that
both
cytoplasmic and mitochondrial CK isoforms arose prior
to the point of
echinoderm-chordate divergence.
Arginine kinase
is widely distributed in the lower and higher
invertebrate groups including echinoderms, is present
in many lower
chordates but is absent in the vertebrates. Conventional
wisdom would
suggest that AK is the most primitive member of the phosphagen
kinase
family and that the other members, including CK, arose
from tandem gene
duplications and subsequent divergence. This perspective
is based on AKs
use of the amino acid arginine rather than some secondarily
derived
guanidine compound as substrate, its presence in the
bulk of the
invertebrate phyla including the most primitive groups
and, lastly, the fact
that AK is generally found as 40 kDa monomers in contrast
to other
phosphagen kinases which typically are multimers. Recently,
a crystal
structure for the transition state analog complex (TSAC)
of the monomeric
AK from the horseshoe crab has appeared (Fig.2).
もくじ