Upon binding calcium, helix 3 of S100A1 re-orients from being relatively antiparallel to helix 4 to being roughly perpendicular. This conformational change is different from most EF hands, in that the entering helix, and not the exiting helix, moves. This conformational change exposes a large hydrophobic pocket between helix 3, 4, and the hinge region of S100A1 that is involved in virtually all calcium-dependent target protein interactions. These biophysical properties seem to be well conserved across the S100 family of proteins. Helix 3, 4, and the hinge region are the most divergent areas between individual S100 proteins, and so it is likely that the sequence of these regions is pivotal in fine-tuning calcium-dependent target binding by S100 proteins.[10]S-Nitrosylation of S100A1 at Cys85 reorganizes the conformation of S100A1 at the C-terminal helix and the linker connecting the two EF hand domains.[11]
The most accurate high-resolution solution structure of human apo-S100A1 protein (PDB accession code: 2L0P) has been determined by means of NMR spectroscopy in 2011.[12]
S100 genes include at least 19 members which are located as a cluster on chromosome 1q21.[13][14]
Function
S100 proteins are localized in the cytoplasm and/or nucleus of a wide range of cells, and involved in the regulation of a number of cellular processes such as cell cycle progression and differentiation. This protein may function in stimulation of Ca2+-induced Ca2+ release, inhibition of microtubule assembly, and inhibition of protein kinase C-mediated phosphorylation.
S100A1 is expressed during development in the primitive heart at embryonic day 8 in levels that are similar between atria and ventricles. As development progresses up to embryonic day 17.5, S100A1 expression shifts to a lower levels in atria and higher levels in ventricularmyocardium.[15]
S100A1 has also been identified as a novel regulator of endothelial cell post-ischemic angiogenesis, as patients with limb ischemia exhibited downregulation of S100A1 expression in hypoxic tissue.[32][33]
S100A1 has shown efficacy in feasibility in treating heart failure symptoms in large, preclinical models and human cardiomyocytes,[35][36] and thus shows great promise for clinical trials.[37][38][39][40][41][42][43]
Reduced expression of this protein has been implicated in cardiomyopathies,[44] and left ventricular assist device-based therapy does not restore S100A1 levels in patients.[45] S100A1 has shown promise as an early diagnostic biomarker for acute myocardial ischemia, presenting with a distinct timecourse in human plasma following an ischemic event relative to traditional markers creatine kinase, CKMB and troponin I.[46][47] This injury-released, extracellular pool of S100A1 was investigated in neonatal murine cardiomyocytes and was shown to prevent apoptosis via an ERK1/2-dependent pathway, suggesting that the release of S100A1 from injured cells is an intrinsic survival mechanism for viable myocardium.[48] S100 has also shown promise as a biomarker for uncontrolled hyperoxic reoxygenation during cardiopulmonary bypass in infants with cyanotic heart disease[49] and in adults.[50] S100A1 gene transfer to engineered heart tissue was shown to augment contractile performance of the tissue implants, suggesting that S100A1 may be effective in facilitating cardiac tissue replacement therapy in heart failure patients.[51] However, the clinical efficacy of this strategy remains to be determined.
In addition, multiple drugs, including Pentamidine,[10]Amlexanox, Olopatadine, Cromolyn, and Propranolol,[10] are known to bind to S100A1, although their affinities are often in the mid-micromolar range.
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^Engelkamp D, Schäfer BW, Erne P, Heizmann CW (October 1992). "S100 alpha, CAPL, and CACY: molecular cloning and expression analysis of three calcium-binding proteins from human heart". Biochemistry. 31 (42): 10258–64. doi:10.1021/bi00157a012. PMID1384693.
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