Calcium Carrier protein

Second, albumin is a non-specific carrier protein. A wide range of chemically disparate compounds are bound loosely to albumin for transport through the blood stream. Important examples include calcium, bilirubin, drugs and free fatty acids. 

Based on these and previous data, the following mineralization mechanism is proposed. One protein fraction which we will term the Carrier Protein (CP), is secreted by the organism and will polymerize in the form of sheets. It serves as a carrier and binding agent for an acidic polypeptide fraction which has a strong affinity to calcium ions. This polypeptide will be designated as the Mineralization Matrix (MM). 

Calcium transfer observed in chick embryo may also be controlled by a similar mechanism since the newly hatched chick has six times the amount of calcium initially present in the egg yolk. In this case the egg shell is the bound Ca2+ pool. It is not clear to what extent calcium-activated ATPase or a calcium-binding protein carrier is involved in active transcellular transport of calcium277,278. ... 

Wilder PT, Lin J, Bair CL, Charpentier TH, Yang D, Liriano M, Varney KM, Lee A, Oppenheim AB, Adhya S, Carrier F, Weber DJ. Recognition of the tumor suppressor protein p53 and other protein targets by the calcium-binding protein S100B. Biochim Biophys Acta - Molecular Cell Research 2006, in press. 

In summary, one might expect to find calcium-binding proteins playing six distinct roles in living systems 1. binding sites on the outer surface of plasma membranes, 2. transport carriers in cell membranes, 3. intracellular storage reservoirs of calcium, 4. intracellular receptors linked to calcium function (i.e., in contractile systems), 5. as part of matrix of mineralized tissues and, 6. as a co-factor in calcium-activated enzymes. 

Cystinuria is the prototype of a number of inborn errors of metabolism believed to result from the deficiency of carrier proteins involved in transporting molecules through the cell membrane. Transport defects in kidney, intestine, or both have also been described for other amino acids— glycine, cystine, tryptophan, methionine—for glucose and galactose, and even for electrolytes such as calcium, chloride, and sodium [173]. Some of these inborn errors are described in other chapters. 

The principal biologically active natural forms of pantothenic acid (vitamin B5), one of the B vitamin complex, are coenzyme A (CoA or CoASH) and acyl-carrier protein (ACP). In solid pharmaceuticals, foods, and feeds, because of simple handling and increased stability, the pantothenic acid sodium and calcium salt are often used as additives. Panthenol is usually used in liquid pharmaceutical preparations and in cosmetics. 

Vitamin B5 occurs in three biologically active forms in foods [1] pantothenic acid, coenzyme A (CoA), and acyl carrier protein (ACP). Calcium or sodium pantothenate are the forms generally used as supplements in infant formula [4], The total quantification of vitamin B5 requires the release of pantothenic acid from CoA and ACR Since it consists of pantoic acid linked through an amide linkage to p-alanine, chemical hydrolysis cannot be used. The only alternative to free pantothenic acid from CoA is the digestion with a number of enzymes (pepsin, alkaline phosphatase, pantetheinase) nevertheless, this treatment is unable to release the vitamin from ACP [27,28]. For the extraction of free pantothenic acid from milk and calcium pantothenate from infant formula an acidic deproteination is often used, followed by centrifugation and filtration [29,30]. 

Minerals and Vitamins. Mineral absorption occurs throughout the small and laige intestines, with the rate of absorption depending on a number of factors—pH, carriers, diet composition, etc. Numerous mechanisms of mineral absorption have been elucidated. Many minerals, for example, iron and sodium, require active transport systems. Others, such as calcium, utilize both carrier proteins and diffusion mechanisms. Moreover, vitamin D is required for calcium absorption, and vitamins C and E favor the absorption of iron. 

In both cases the hormone is bound to neurophysin. Nerve stimuli arrive at the terminals in the neural lobe. In the depolarization procedure calcium enters the axoplasm or is released from subcellular sites. The slight increase in the intracellular concentration of calcium ions causes a release of octapeptide from binding to the carrier protein. 

Schematic illustration of a generalized cholinergic junction (not to scale). Choline is transported into the presynaptic nerve terminal by a sodium-dependent choline transporter (CHT). This transporter can be inhibited by hemicholinium drugs. In the cytoplasm, acetylcholine is synthesized from choline and acetyl -A (AcCoA) by the enzyme choline acetyltransferase (ChAT). Acetylcholine is then transported into the storage vesicle by a second carrier, the vesicle-associated transporter (VAT), which can be inhibited by vesamicol. Peptides (P), adenosine triphosphate (ATP), and proteoglycan are also stored in the vesicle. Release of transmitter occurs when voltage-sensitive calcium channels in the terminal membrane are opened, allowing an influx of calcium. The resulting increase in intracellular calcium causes fusion of vesicles with the surface membrane and exocytotic expulsion of acetylcholine and cotransmitters into the junctional cleft (see text). This step can he blocked by botulinum toxin. Acetylcholine s action is terminated by metabolism by the enzyme acetylcholinesterase. Receptors on the presynaptic nerve ending modulate transmitter release. SNAPs, synaptosome-associated proteins VAMPs, vesicle-associated membrane proteins.

The carrier-mediated active transport system of calcium is responsible for the relaxation of muscle. However, the rate of efflux from sarcoplasmic reticulum membranes during reversal of the transport process is 102 to 104 orders too low to account for the massive calcium release from sarcoplasmic reticulum in stimulated muscle. Instead, passive diffusion of calcium across the sarcoplasmic reticulum membrane will proceed during excitation of muscle178,179,186. The rate of calcium release observed during excitation is 1.000-3.000 p moles/mg protein/min which is an increase of about 104 to 10s over the resting state. 

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