CCDC138

 

Molecular Structure

CCDC138 is a soluble protein that contains multiple alpha helixes and beta sheets, as well as a domain of unknown function. It is predicted by PELE, CHOFAS, and GOR4 to form a number of coiled-coils and helices.

Hemoglobins are characterized by the presence of oxygen-binding sites on their beta polypeptide chains. Over 400 hemoglobin variants exist in humans, resulting from amino acid substitutions on either the alpha or beta polypeptide chains. Most of these amino acid substitutions have no effect on the cellular oxygen-binding properties of hemoglobin, but some of them do, Bet  and their locations can be pinpointed using electrospray ionization collisionally activated dissociation (ESI-CAD) mass spectrometry.

For example, the Brockton variant, which is associated with anemia, has a proline-to-asparagine substitution at position 138 of its beta chain. Hemoglobins with this substitution have the same electrophoretic mobility as Hb A, but their oxygen affinity is much lower. The instability of this hemoglobin is probably due to the loss of a critical buried hydrogen bond at beta 138 and Val 134 of its beta carboxyl-terminal dipeptide.

Oxygen Affinity

The oxygen affinity of hemoglobin (P50) is a physiologically important feature that influences the distribution of blood, and consequently the amount of oxygen available to tissues. Hemoglobin can bind and release oxygen in either the relaxed, high-oxygen affinity R state or the tense, low-oxygen-affinity taut T state. This is a cooperative process, and the binding of oxygen to hemoglobin is dependent on pH, with acidosis decreasing P50 by the Bohr effect.

The P50 of hemoglobin can be altered by mutations that cause stabilization of the R state or destabilization of the T state. Four of these mutations (Hb Kansas, Hb Beth Israel, Hb Richmond and Hb Saint Mande) affect a critical residue at position 102 in the helix of hemin chain B.

Variants with altered oxygen affinity can be distinguished from normal Hb A by analyzing blood or hemolysate with the p50 test. Most of these variants can also be separated from Hb A by electrophoresis or chromatography.

Electrophoretic Mobility

Electrophoretic mobility is the average velocity of charged species through a given medium under an electric field. This is a property of ions that is important in electrophoretic separations such as those used for protein and nucleic acid electrophoresis.

The average velocity of cations (positively charged) is greater than that of anions (negatively charged) because the electric field causes an electroosmotic flow that tends to separate cations from anions. This is why the typical instrumental arrangement involves putting the negative electrode at the injection end of the capillary, and assigning the positive electrode at the detection end.

The electrophoretic injection is biased toward solutes with larger electrophoretic mobilities - in other words, towards the solute that has a higher limiting ionic conductivity (equivalent to the sum of the buffer and sample conductivities). This improves efficiency but reduces resolution.

Stability

Hemoglobin Brockton (beta 138 (H16) Ala----Pro) was identified in two unrelated black families with mild anemia. This unstable variant is not resolved from Hb F on cellulose acetate electrophoresis, although it does have the same oxygen affinity as normal hemoglobin and no biphasic oxygen saturation in blood or hemolysate. The instability probably results from the inability of a buried hydrogen bond to form between Pro 138 beta and Val 134 beta of the helix-turn-helix region, which is essential for the proper oxygen binding properties of hemoglobin.

The expression of p21 and p16, members of the tumour suppressor pathways regulated by p53, increases with cell senescence. Therefore, their expression can be used to identify senescent cells in tissue and cultured cells. These cells are characterized by permanent cell cycle arrest and accumulation of senescence-associated chromatin modifications.