Minerals on the Move
Transport in the Blood and
Cellular Uptake
Postabsorption (postprandial) events
in mineral nutrition
1. Transport in the blood
2. Movement across membranes
3. Intracellular location
Rules to Consider
Rule: Whereas macrominerals (Ca2+, Mg2+, Na+, Cl- etc.) travel in the
blood and access cells primarily as free ions, the micronutrients (Cu2+,
Zn2+, Fe2+, Mn2,, Se) rely on proteins and other ligands for transport
and delivery
Rule: Targeting microminerals to select organs and locations within cells
is a function of transport proteins in concert with low Mwt ligands.
Rule: Common to both macro-and microminerals are specific portals
or gateways to channels in the membranes through which minerals
can move from outside to inside the cell. The activity of these portals
is carefully regulated.
Rule: In general, because of their bulk, macrominerals use the energy of
diffusion to gain access to the cytosol from the extracellular fluid,
microminerals require energy-driven transport mechanisms.
Major Blood Minerals
Total
(mmol/L)
Calcium
2.5
Bound
(mmol/L)
% Bound
1.0 – 1.5
46
Magnesium
0.48-0.66
0.1 – 0.3
32
Potassium
3.5 – 5.0
minor
_
135 - 145
minor
_
Sodium
Chloride
Phosphate
98 - 108
0.7 – 1.4
minor
0.5 – 1.0
_
70 (phospholipids)
Transport in the Blood
The objective of blood transport is to relocate an absorbed
mineral to its action site or storage site
Transport proteins generally work with receptors on cell membranes
A variety of transport carriers have been identified for both macroand microminerals
Fe (non heme)
Transferrin
Zn
-2 macroglobulin, albumin
Cu
Ceruloplasmin, serum albumin
Mn
Transferrin
Ca
Albumin (50% protein bound)
Mg
Albumin (32% protein bound)
Se
Selenoprotein P
Table 1. Distribution and Kinetics of Body Iron
Compartment
Iron (grams) Percent of Total
Hemoglobin
2.7
66
Myoglobin
0.2
3
0.008
0.1
Non-heme Enzymes
< 0.0001
---
Intracellular Storage
(Ferritin)
1.0
30
Intracellular Labile Iron
(Chelatable Iron)
0.07 (?)
1
Intercellular Transport
(Transferrin)
0.003
0.1
Heme Enzymes
Total
3.98 grams
Typical transport protein
Membrane receptor recognition site
Metal-binding sites
Revealing a secret molecular handshake. A new
model shows that iron-transporting transferrins bind
to the side and underside of the transferrin
receptor, not to the top. The transferrin molecule
straightens when it attaches to the receptor, which
puts the C-lobe in a position to more quickly
release its iron load, while the N-lobe may have a
harder time letting go of its metal cargo.
Transferrin iron enters via endocytosis
Receptor recognitionbinding site
Transmembrane Transport
Membrane receptors
Gated channels
ATP-driven transport
Endocytosis
4 ways Minerals penetrate
the Cell Membrane
• Simple Diffusion
• Passive transport (facilitated
diffusion)
• Active transport (energy-dependent)
• Receptor-mediated endocytosis
Simple Diffusion
Initial
Final
High
Low
Ficks Law of diffusion: The rate of diffusion of an ion at steadystate transmembrane flux varies inversely with path length and
directly with area and concentration gradient
Unbound Ca = 1.0 mM
ADca
in blood
([Ca]1 – [Ca]2)
F=
L
Cytosolic Ca = 0.00001 mM
2
A = 80 m
L = 10 m
Dca = 3 x 10-3 cm2/min
after Bronner
Adolph Fick
Based on Fick’s law, the expected diffusion rate of Ca
across the intestinal cell is 96 x 10-18 mol/min/cell.
Rate observed is 70 times greater at Vmax, which means duodenal
cells have factors that enhance self diffusion of Ca
Factor identified as Calbindin, a small (9 kD) Ca-binding protein
Facilitated Diffusion (Mediated Transport)
Calcium Channels
Topology of a calcium channel sitting in the cell membrane. Just a mutation in
one of the 2,000-plus amino acids (red dot) disrupts the molecule's shut-off
mechanism and allows abnormal calcium entry into cells.
A molecular model of a calcium channel protein (purple
helices) complexed with the drug nifedipine in the middle
of the pore. The orange spheres are isoleucine residues
that act as a "gate" for the passage of calcium ions (yellow
spheres). Drugs may close the gate and prevent the influx
of calcium. Such channel-blocking drugs help reduce
muscle contraction and are widely used in treating cardiac
disorders.
Potassium Channels
ION FUNNEL. This side view of a
potassium channel reveals its invertedteepee shape embedded in a cell
membrane with the extracellular side
facing up and the cytoplasmic side down.
TIGHT FIT. This view of the potassium channel
shows the channel's four identical subunits in
different colors. The center of the channel holds a
potassium ion (green).
ATP-Driven
(Active) Transport
[Ca2+-ATPase] for removing Ca2+ from cytosol
Cytosol
Extracellular
Selecting a specific cell
is a function of a
membrane receptor
In the case of transferrin iron,
the major route of entry is via
receptor-mediated
endocytosis
Receptor-Mediated Endocytosis
Structural view of the role of the
hemochromatosis protein HFE, a class I
major histocompatibility complex (MHC)
homolog, in regulating iron uptake by
cells. Foreground: two HFE molecules
(red and orange ribbons) bound to a
homodimeric transferrin receptor (blue
ribbons) as seen in the 2.8 angstrom
crystal structure of an HFE-transferrin
receptor complex. Background: HFE
competes with transferrin (green) for
binding to transferrin receptor.
Localization of calbindin in
microvilli and cytosolic
vesicles
Structural response to calcium in calbindin D9k and
calmodulin. Apo calbindin D9k is shown in dark blue. Calciumloaded calbindin D9k is shown in red. Apo N-terminal domain of
calmodulin is shown in light blue. Calcium-loaded N-terminal
domain of calmodulin is shown in pink. As the picture indicates,
the calcium-induced conformational changes are much more
pronounced in the calmodulin domains than in calbindin D9k.
Intracellular Movement is a function of
proteins, low molecular weight ligands
and vesicles.
1. The metallochaperones that conduct copper localization
2. Calbindin transfers calcium
3. Glutathione postulated to work with copper
Copper
Valeria Culotta
Undetectable Intracellular Free COPPER: The Requirement of a
COPPER Chaperone for Superoxide Dismutase
T. D. Rae, 1 P. J. Schmidt, 3 R. A. Pufahl, 1 V. C. Culotta, 3* T. V.
O'Halloran 12*
The COPPER chaperone for the superoxide dismutase (CCS) gene is
necessary for expression of an active, COPPER-bound form of
superoxide dismutase (SOD1). In vitro studies demonstrated that
purified Cu(I)-CCS protein is is necessary only when the concentration of
free COPPER ions ([Cu]free) is strictly limited. Moreover, the
physiological requirement for CCS in vivo was readily bypassed by
elevated COPPER concentrations. This metallochaperone protein
activates the target enzyme through direct insertion of the COPPER
cofactor and apparently functions to protect the metal ion from binding to
intracellular COPPER scavengers. These results indicate that
intracellular [Cu]free is limited to less than one free COPPER ion per cell
and suggest that a pool of free COPPER ions is not used in physiological
activation of metalloenzymes.
Typical Copper Chaperone
Hereditary Hemochromatosis (HH)
Hepcidin (RR, RE)
(Master regulator of iron homeostasis)
Positive regulator (+)
Intestinal absorption
Level
Positive regulator (+)
HFE (RR)
Hemojuvelin (HJV) RR
(coded by HFE2 gene)
Cellular uptake level
Transferrin receptor (RE)
Transferrin (RR)
Suppresses (-)
Ferroportin (RE)
Excess Hepcidin
= Anemia
Defective Hepcidin = Hemochromatosis (HH)
RR = regulator
RE = regulator target
Defective HJV
= low Hepcidin (HH)
Defective HFE
= low Hepcidin (HH)
excess transferrin iron
uptake