Trinuclear Calcium Site in the C2 Domain of PKCα/γ Is Prone to Lithium Attack

How Theoretical Evaluations Can Generate Guidelines for Designing/Engineering Metalloproteins with Desired Metal Affinity and Selectivity

Almost half of all known proteins contain metal co-factors. Crucial for the flawless performance of a metalloprotein is the selection with high fidelity of the cognate metal cation from the surrounding biological fluids. Therefore, elucidating the factors controlling the metal binding and selectivity in metalloproteins is of particular significance. The knowledge thus acquired not only contributes to better understanding of the intimate mechanism of these events but, also, significantly enriches the researcher's toolbox that could be used in designing/engineering novel metalloprotein structures with pre-programmed properties. A powerful tool in aid of deciphering the physical principles behind the processes of metal recognition and selectivity is theoretical modeling of metal-containing biological structures. This review summarizes recent findings in the field with an emphasis on elucidating the major factors governing these processes. The results from theoretical evaluations are discussed. It is the hope that the physical principles evaluated can serve as guidelines in designing/engineering of novel metalloproteins of interest to both science and industry.

Factors allowing small monovalent Li+ to displace Ca2+ in proteins

Because Li⁺ and Ca²⁺ differ in both charge and size, the possibility that monovalent Li⁺ could dislodge the bulkier, divalent Ca²⁺ in Ca²⁺ proteins had not been considered. However, our recent density functional theory/continuum dielectric calculations predicted that Li⁺ could displace the native Ca²⁺ from the C2 domain of cytosolic PKCα/γ. This would reduce electrostatic interactions between the Li⁺-bound C2 domain and the membrane, consistent with experimental studies showing that Li⁺ can inhibit the translocation of cytoplasmic PKC to membranes. Besides the trinuclear Ca²⁺-site in the PKCα/γ C2 domain, it is not known whether other Ca²⁺-sites in human proteins may be susceptible to Li⁺ substitution. Furthermore, it is unclear what factors determine the outcome of the competition between divalent Ca²⁺ and monovalent Li⁺. Here we show that the net charge of residues in the first and second coordination shell is a key determinant of the selectivity for divalent Ca²⁺ over monovalent Li⁺ in proteins: neutral/anionic Ca²⁺-carboxylate sites are protected against Li⁺ attack. They are further protected by outer-shell Asp^-/Glu^- and the protein matrix rigidifying the Ca²⁺-site or limiting water entry. In contrast, buried, cationic Ca²⁺-sites surrounded by Arg⁺/Lys⁺, which are found in the C2 domains of PKCα/γ, as well as certain synaptotagmins, are prone to Li⁺ attack.

Ca2+/Sr2+ Selectivity in Calcium-Sensing Receptor (CaSR): Implications for Strontium's Anti-Osteoporosis Effect

The extracellular calcium-sensing receptor (CaSR) controls vital bone cell functions such as cell growth, differentiation and apoptosis. The binding of the native agonist (Ca²⁺) to CaSR activates the receptor, which undergoes structural changes that trigger a cascade of events along the cellular signaling pathways. Strontium (in the form of soluble salts) has been found to also be a CaSR agonist. The activation of the receptor by Sr²⁺ is considered to be the major mechanism through which strontium exerts its anti-osteoporosis effect, mostly in postmenopausal women. Strontium-activated CaSR initiates a series of signal transduction events resulting in both osteoclast apoptosis and osteoblast differentiation, thus strengthening the bone tissue. The intimate mechanism of Sr²⁺ activation of CaSR is still enigmatic. Herewith, by employing a combination of density functional theory (DFT) calculations and polarizable continuum model (PCM) computations, we have found that the Ca²⁺ binding sites 1, 3, and 4 in the activated CaSR, although possessing a different number and type of protein ligands, overall structure and charge state, are all selective for Ca²⁺ over Sr²⁺. The three binding sites, regardless of their structural differences, exhibit almost equal metal selectivity if they are flexible and have no geometrical constraints on the incoming Sr²⁺. In contrast to Ca²⁺ and Sr²⁺, Mg²⁺ constructs, when allowed to fully relax during the optimization process, adopt their stringent six-coordinated octahedral structure at the expense of detaching a one-backbone carbonyl ligand and shifting it to the second coordination layer of the metal. The binding of Mg²⁺ and Sr²⁺ to a rigid/inflexible calcium-designed binding pocket requires an additional energy penalty for the binding ion; however, the price for doing so (to be paid by Sr²⁺) is much less than that of Mg²⁺. The results obtained delineate the key factors controlling the competition between metal cations for the receptor and shed light on some aspects of strontium’s therapeutic effects.

Calcium in Signaling: Its Specificity and Vulnerabilities toward Biogenic and Abiogenic Metal Ions

Divalent calcium ion (Ca²⁺) plays an indispensable role as a second messenger in a myriad of signal transduction processes. Of utmost importance for the faultless functioning of calcium-modulated signaling proteins is their binding selectivity of the native metal cation over rival biogenic/abiogenic metal ion contenders in the intra/extracellular fluids. In this Perspective, we summarize recent findings on the competition between the cognate Ca²⁺ and other biogenic or abiogenic divalent cations for binding to Ca²⁺-signaling proteins or organic cofactors. We describe the competition between the two most abundant intracellular biogenic metal ions (Mg²⁺ and Ca²⁺) for Ca²⁺-binding sites in signaling proteins, followed by the rivalry between native Ca²⁺ and "therapeutic" Li⁺ as well as "toxic" Pb²⁺. We delineate the key factors governing the rivalry between the native and non-native cations in proteins and highlight key implications for the biological performance of the respective proteins/organic cofactors.