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

                                                            

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: 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.

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.

Rules to Consider

Major Blood Minerals

Calcium

Magnesium 0.48-0.66

Total(mmol/L)

Bound(mmol/L)

0.1 – 0.3

2.5 1.0 – 1.5

% Bound

46

32

Potassium 3.5 – 5.0

Sodium 135 - 145

Chloride 98 - 108

minor

minor

minor

Phosphate 0.7 – 1.4 70 (phospholipids)

_

_

_

0.5 – 1.0

Transport in the Blood

A variety of transport carriers have been identified for both macro- and microminerals

The objective of blood transport is to relocate an absorbed mineral to its action site or storage site

Fe (non heme)

Zn

Cu

Mn

Transferrin

-2 macroglobulin, albumin

Ceruloplasmin, serum albumin

Transferrin

Ca

Mg

Transport proteins generally work with receptors on cell membranes

Albumin (50% protein bound)

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

Heme Enzymes 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 

Total 3.98 grams

Typical transport protein

Metal-binding sites

Membrane receptor recognition site

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 recognition-binding 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 DiffusionInitial

High

Low

Final

Ficks Law of diffusion: The rate of diffusion of an ion at steady-state transmembrane flux varies inversely with path length and directly with area and concentration gradient

F = ADca

L([Ca]1 – [Ca]2)

A = 80 m2

L = 10 m

Dca = 3 x 10-3 cm2/min

Based on Fick’s law, the expected diffusion rate of Ca across the intestinal cell is 96 x 10-18 mol/min/cell.

Adolph Fick

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

after Bronner

Unbound Ca = 1.0 mM in blood

Cytosolic Ca = 0.00001 mM

Facilitated Diffusion (Mediated Transport)

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.

Calcium Channels

                                

ION FUNNEL. This side view of a potassium channel reveals its inverted-teepee 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).

Potassium Channels

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.

Structural response to calcium in calbindin D9k and calmodulin. Apo calbindin D9k is shown in dark blue. Calcium-loaded 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.

Localization of calbindin in microvilli and cytosolic

vesicles

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 Valeria Culotta

Copper

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

                                                                                                                                                                                                                                                                                                   

                                             

Hepcidin (RR, RE)(Master regulator of iron homeostasis)

HFE (RR)

Transferrin receptor (RE)

Hereditary Hemochromatosis (HH)

Hemojuvelin (HJV) RR

Transferrin (RR)

Ferroportin (RE)

Suppresses (-)

Excess Hepcidin = Anemia

Defective Hepcidin = Hemochromatosis (HH)

Positive regulator (+)

Defective HJV = low Hepcidin (HH)

(coded by HFE2 gene)

Positive regulator (+)

Defective HFE = low Hepcidin (HH) excess transferrin iron uptake

Cellular uptake level

Intestinal absorption Level

RR = regulatorRE = regulator target