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