microreview
Energy transduction associated with ion gradients.
ATP synthase couples proton escape from the high-[H+]
intermembrane space of mitochondria or chloroplasts to
rotation of a central stalk that in turn applies mechanical
force to ADP + Pi to generate ATP in one of three active
sites, one after another.
Bateriorhodopsin captures the energy from light as strain in
a retinal cofactor and then couples return to the groundstate isomer to transfer of one H+ (or OH-) up its
concentration gradient.
Halorhodopsin is homologous to BR and executes lightdriven Cl- pumping.
A.-F. Miller, 2008, pg
1
Mg2+, in brief
Small, divalent, octahedral (6 ligands in the centres of the 6
faces of a cube).
Mg2+ and Na+ both prefer 6-coordination whereas K+ and
Ca2+ can expand their coordination sphere to 7 or 8
ligands.
Because of the small size of Mg2+’s coordination sphere
(short bonds), it typically only has a few ligand from protein,
and several ligands are usually waters (small and
accommodating of the other ligands’ positions).
Table 10.1
A.-F. Miller, 2008, pg
2
Mg2+ in structure and catalysis
Mg2+’s charge per unit volume is very high and the ligand
exchange rate is correspondingly slow.
It is often involved in nucleotide and nucleic acid chemistry
as a long-lived complex with ATP or ADP. The true substrate
for most kinases is not ATP but ATP•Mg.
Mg2+ is also a component of numerous enzymes. Its most
common catalytic role is that of a Lewis acid: it stabilizes
deprotonated phosphoryl sites, and OH-, which are thus
activated as nucleophiles relative to phosphates and H2O.
Stabilization of enolate intermediates that are central the
mechanisms of numerous enzymes in carbohydrate
degradation and transformations. See Figure 10.10.
A.-F. Miller, 2008, pg
3
Phosphoryl transfer
Reactions crucial to central energy metabolism, and to
signalling cascades.
Phosphorylation of sugars. At the top of the metabolic
pathway for catabolism.
Phosphorylation of proteins activates some, others are
inactivated. Why are phosphates the signals of choice ?
Are they ?
Highly negatively charged nucleic acids and nucleotides
require complexation with Mg2+.
A.-F. Miller, 2008, pg
4
Mg2+ in chlorophyll
Mg2+ is in the centre of the pigment cycle, where it
modulates the absorbance maximum of the pigment, and
thus the energy of photons that it captures.
A pair of overlapping parallel chlorophylls is the site of
electron transfer in response to capture of a photon,
they convert light energy into the early steps of chemical
energy generation.
A.-F. Miller, 2008, pg
5
Ca2+ homeostasis
Ionic radius of 0.95 Å is much bigger than that of Mg2+ (0.6 Å).
More relaxed coordination preferences, often more than 6
ligands, and distances of 2.3 - 2.8 Å (vs. Oh Mg+ with 6 ligands at
2.05 Å).
Kinetics of ligand exchange are fast (1010 s-1 vs. 106 s-1 for Mg2+).
Biominerals account for 99 % of body Ca2+.
The remaining 1 % is crucial nonetheless.
Ligand-gated Ca2+ channels are involved in neuronal signalling
(eg. in response to MSG). NMDA (N-methyl d-acetate) is an
agonist, hence the name of these channels.
A.-F. Miller, 2008, pg
6
2+
Ca
Special role
originating
in low
solubility of
several Ca
salts ?
A.-F. Miller, 2008, pg
7
stores
Activation of
mitochondrial
metabolic
activity by Ca
from the ER
A.-F. Miller, 2008, pg
8
2+
Ca
and Cell signalling
Signalling, via enzyme activity and structural stability.
Second messenger (eg. Ca2+)
Second messengers carry signals initially detected by
receptors embedded in membranes of the cell or
compartment.
Second messengers activate enzymes or cascades of
enzymes: amplify the signal.
Three families of receptors: G-protein-coupled receptors,
single transmembrane segment catalytic receptors (initiate
phosphorylation and de~), oligomeric ion channels activated
by2008,
conformational
changes.
A.-F. Miller,
pg
9
Stage 1: G-protein-coupled receptors
Homologous to BR: 7 transmembrane helices.
β-adrenergic receptor employs a buried conserved Asp in the
3rd TMH to bind cationic ligands (catecholamines,
epinephrine).
Upon ligand binding, a conformational change is propagated to
inside of membrane where it activates a G-protein.
Activated G-proteins (of which there are multiple kinds)
trigger second messenger production by formation of cAMP,
activation of phospholipases that release phospholipid-derived
second messengers and activation of Ca2+ and K+ channels.
A.-F. Miller, 2008, pg
10
GPCR activation of adenylate
cyclase
A.-F. Miller, 2008, pg
11
Garrett & Grisham Fig. 32.10
Second messengers ( inc. Ca2+)
cAMP is one of many second messengers that propagate the
response and diversity it.
Inositol phosphates and diacyl glycerols propagate the
message too.
A.-F. Miller, 2008, pg
12
GPCR-activated phospholipases
Different phospholipases release different second messengers.
Garrett & Grisham Fig. 32.16
Prostoglandins
A.-F. Miller, 2008, pg
13
Products include phosphoinositols,
phosphoinositol phosphates (up to IP3),
diacyl glycerols (DAG).
Many of the phospholipases are
Ca2+-dependent
A.-F. Miller, 2008, pg
Garrett & Grisham Fig. 32.18
14
Hormone-activated Ca2+ channels.
Resulting increases in intracellular [Ca2+] activate various
processes: muscle contraction, exocytosis, glycogen
metabolism, membrane dissolution.
Channels in the plasma
membrane can open as a
response to a signal from
cAMP. Intracellular
reserves such as
reticulum and calciosomes
are tapped via an IP3
message.
A.-F. Miller, 2008, pg
15
Garrett & Grisham Fig. 32.21
Proteins that mediate Ca2+ signals
Phospholipase C
Ca2+-modulated proteins (via calmodulin: troponin C)
Annexins
Ca2+-modulated proteins: a feature of metabolically active
tissues: highly active neurons.
Signature ‘EF hands’ comprised of Glu,
3Asp, bkbn O (6Os) and a coordinated
water), found in calmodulin, troponin C,
calbindin.
Two EF hands commonly interact with
one another once they are Ca2+-bound.
A.-F. Miller, 2008, pg
16
Effector binding.
CaM-binding region
of spectrin
Target proteins are many and diverse, but all have a basic amphipathic α helix.
CaM binds with nM-pM affinities by bending around and matching its large
hydrophobic surfaces flanked by negative electrostatic regions around the
helix.
Garrett & Grisham Fig. 32.24
A.-F. Miller, 2008, pg
17