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Calmodulin (CaM) is a calcium-binding protein in eukaryotes that controls various calcium-regulated cellular processes. It has four calcium-binding sites that bind calcium to form the calcium-calmodulin ( Ca2+-CaM) complex. GPCR stimulation increases the calcium levels in the cells that bind to CaM and induces a conformational change.
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Calcium binding by spinach stromal proteins.

G Kreimer1, B Surek, I E Woodrow

  • 1Botanisches Institut der Westfälischen Wilhelms-Universität, Schloßgarten 3, D-4400, Münster, Federal Republic of Germany.

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Summary

Spinach stromal proteins exhibit calcium binding with multiple sites and varying affinities. Changes in pH significantly impact calcium binding, influencing stromal free calcium levels between light and dark conditions.

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Area of Science:

  • Plant biochemistry
  • Photosynthesis research
  • Molecular plant physiology

Background:

  • Calcium ions (Ca2+) play crucial roles in plant cell signaling and photosynthesis.
  • Understanding calcium binding to stromal proteins is essential for elucidating photosynthetic regulation.
  • Previous studies have investigated calcium binding in thylakoid membranes, but stromal interactions require further clarification.

Purpose of the Study:

  • To quantify calcium binding to spinach stromal proteins.
  • To characterize the binding sites and affinities for calcium.
  • To investigate the influence of environmental factors (pH, other ions) on calcium binding and its physiological relevance in photosynthesis.

Main Methods:

  • Dual-wavelength spectrophotometry utilizing the metallochromic indicator tetramethylmurexide.
  • Competitive inhibition assays with magnesium (Mg2+) and lanthanum (La3+) ions.
  • Analysis of calcium binding under varying pH conditions and comparison with existing thylakoid data.

Main Results:

  • Spinach stromal proteins possess at least two independent classes of calcium binding sites.
  • Total binding sites ranged from 90-155 nmol·mg(-1) protein with average binding constants of 1.1-2.7·mM(-1).
  • pH decrease (7.8 to 7.1) reduced binding sites but increased average affinity, leading to higher free Ca2+ in the stroma under dark conditions (pH 7.1) compared to light (pH 7.8).

Conclusions:

  • Calcium binding to spinach stromal proteins is significant and influenced by pH.
  • The study provides evidence for differential free Ca2+ concentrations in the stroma between light and dark conditions.
  • These findings suggest a regulatory role for stromal calcium binding in photosynthetic processes.