Annotations notes – Dephosphorylation of 2,3-bisphosphoglycerate by MIPP expands the regulatory capacity of the Rapoport-Luebering glycolytic shunt.

Annotations extracted from the articleDephosphorylation of 2,3-bisphosphoglycerate by MIPP expands the regulatory capacity of the Rapoport-Luebering glycolytic shunt.

“Is this 2,3-BPG phosphatase activity of DdMipp1 conserved in higher organisms? To answer this question, we prepared recombinant HsMIPP1 with a C-terminalmyc-poly(His) epitope tag by using the P. pastoris expression system (Fig. 4A). ” (Cho et al 2008:5999)

More on the protein itself:

“MIPP1 is located in the erythrocyte plasma membrane, and its active site faces into the cell (24, 25).” (Cho et al 2008:6000)

The main organic phosphate in erythrocyte is 2,3-BPG.

“Moreover, erythrocytes contain 6-7 mM 2,3-BPG (11), and there are no inositol phosphates to compete for the active site (indeed, MIPP1’s role in erythrocytes has previously been a complete mystery).” (Cho et al 2008:6000)

Hydrolization of 2,3-BPG is a serious issue.

“During the weeks that erythrocytes are stored at 4C before transfusion, 2,3-BPG becomes depleted, causing a temporary but clinically significant impairment of oxygen transport (29). We have found that HsMIPP1 hydrolyzes 2,3-BPG at 4C (Fig. 4B), albeit considerably more slowly than at 37C. Therefore, MIPP1 can contribute to the depletion of 2,3-BPG during erythrocyte storage.” (Cho et al 2008:6000)

Bohr-effect and 2,3-BPG are interrelated.

“As hemoglobin releases oxygen, its affinity for H increases, causing intracellular alkalinization (11). This elevated intracellular pH drives a positive feedback loop, increasing levels of 2,3-BPG, thereby facilitating more oxygen release (11). ” (Cho et al 2008:6001)

“(…) suggests that its inhibition might preserve 2,3-BPG levels in erythrocytes stored before transfusion.” (Cho et al 2008:6002)

“induced alkalosis at high altitudes is another situation in which an increased pH is associated with adaptive increases in erythrocyte 2,3-BPG levels (9). However, the mechanisms by which pH affects intracellular levels of 2,3-BPG are not fully understood (11). We now report that the dephosphorylation of 2,3-BPG by MIPP1 is especially sensitive to pH, with enzyme activity decreasing 50% upon elevating pH within the physiological range (33) of 7 to 7.4 (Fig. 4E). Thus, MIPP1 is well suited to the task of adjusting erythrocyte 2,3-BPG levels in response to changes in both pH and tissue oxygen demand.” (Cho et al 2008:6002)

This suggests that speeds of hydrolysis and synthesis are different:

“First, the addition of MIPP1 to the Rapoport-Luebering pathway offers an opportunity for the cell to separately regulate both the synthesis and degradation of 2,3-BPG in ways that are not possible if only a single enzyme, BPGM, were to be solely responsible for both reactions.” (Cho et al 2008:6002)

Other annotations:

“The sensitivity of MIPP1 to physiologically relevant increases in cellular pH (Fig. 4E) provides a mechanistic explanation for the pH-dependent regulation of 2,3-BPG levels in erythrocytes.” (Cho et al 2008:6002)

“As explained in the previous section, this observation considerably improves our understanding of the homeostatic mechanisms that control tissue oxygen delivery. These results are especially significant in that they solve a long-standing puzzle as to the function for human erythrocyte MIPP1 in cells that do not contain any inositol phosphates.” (Cho et al 2008:6002)

“A drug that inhibits MIPP1 may be therapeutically useful by acutely increasing cellular 2,3-BPG levels and improving oxygen delivery to tissues in a number of clinical situations in which oxygen deprivation is a problem.” (Cho et al 2008:6002)

“Our demonstration that HsMIPP1 is active as a 2,3-BPG phosphatase even at 4C (Fig.” (Cho et al 2008:6002)

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