Molecular Mechanisms of Signal Transduction in Heme-Containing Sensor Proteins
Authors: Martínková Markéta 
Published: February 2020

Heme (the iron protoporphyrin IX complex) is one of the most important and extensively characterized cofactors required for proper functioning of many proteins and enzymes. Heme is involved in O2 storage (myoglobin), O2 transfer (hemoglobin), oxygenation (hydroxylation) reactions (cytochromes P450, NO synthase), electron transfer (cytochrome c) and many other functions. In prototype heme-containing proteins, heme itself forms the catalytic or functional center. These proteins are usually composed of one compact structure (one domain). However, recently it was found that heme itself can regulate many physiological processes in many organisms including humans. The novel role of heme is mediated by a group of hemoproteins called heme-containing sensor proteins. Generally, it is known that the heme can act as (i) a preliminary signal of heme sensor proteins and (ii) a gas sensing site of heme-based gas sensor proteins.
Conceptually, these proteins are always composed of at least two domains: one is a sensor domain and the other is a functional domain. However, the structure-function relationship and mechanisms of communication between these domains have not been fully understood. Therefore, we selected model systems from each subfamily of heme-containing sensor proteins - for example, a heme regulated inhibitor, HRI (subfamily i) and a globin-coupled oxygen sensor histidine kinase, AfGcHK (subfamily ii) - in order to study the signal transduction in the novel group of hemoproteins (i.e. heme-containing sensor proteins).
The AfGcHK is a part of the two-component signal transduction system from the soil bacterium Anaeromyxobacter sp. Fw109-5. Once the oxygen molecule (as the first signal) binds to the heme iron complex in the sensor domain of AfGcHK, the functional domain is stimulated, leading to autophosphorylation at a conserved His residue in the functional domain. The phosphate group of phosphorylated AfGcHK is then transferred to the cognate response regulator.
The HRI seems to be one of the most important existing heme sensor proteins for eukaryote survival in response to cell emergency states involved in clinical conditions important for human health. The functional domain of HRI exhibits the kinase activity towards its substrate - eukaryotic initiation factor 2α (eIF2α) depending on the heme availability. The enzyme is active in the absence of heme. The heme binding to the HRI protein induces a global structural change, leading to the heme-induced catalytic inhibition. This sensor protein is important for regulation of eukaryotic proteosynthesis. Eukaryotic cells decrease their overall rates of protein synthesis for survival in response to a variety of stress conditions, such as shortage of amino acids, UV light illumination, viral infection, accumulation of denatured proteins, and shortage of heme. The decrease (inhibition) in protein synthesis is caused by phosphorylation of eIF2α at Ser-51 by eIF2α kinases that respond specifically to stress. HRI is a member of the eIF2α kinase family that controls globin synthesis in response to the heme concentration in reticulocytes to balance the molar ratio of heme and globin. In addition to globin, HRI controls the synthesis of tryptophan 2,3-dioxygenase and cytochrome P450 2B in liver upon acute porphyria.
Several biochemical approaches were utilized in order to study the signal transduction between the sensor and function domains of the model systems: (i) enzyme kinetic study, (ii) hydrogen/deuterium exchange experiments associated with mass spectrometry, (iii) X-ray crystallography and others. Overall our results indicated that the coordination and oxidation state of the sensor domain heme iron affect the enzyme’s catalytic activity of the function domain. The possible contact area between sensor and function domains was also reveled. All these data together will be discussed in order to illustrate the mechanism of signal transduction in the heme-containing sensor proteins.
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