Hemoglobin

Hemoglobin (Hb) is a large molecule (molecular weight = 64,485) consisting of four individual polypep-tide chains, each with a heme (iron-containing) protein that can bind O2 with iron in the ferrous (Fe2+) form. Methemoglobin occurs when the iron is in the ferric form (Fe3+), and it cannot bind O2. Small amounts of methemoglobin normally occur in blood and slightly reduce the amount of O2 that can be bound to hemoglobin. One gram of pure human hemoglobin can bind 1.39 mL of O2 when fully saturated, but methemoglobin reduces this value to 1.34-1.36. Hemoglobin is concentrated inside red blood cells, or erythro-cytes. This cellular packaging is important for the biophysics of the microcirculation, and it provides physiologic control of O2 binding through cellular changes in the hemoglobin microenvironment.

The four subunits of hemoglobin include two alpha and two beta chains, and variations in the amino acid sequence of these polypeptides explain the differences in Hb-O2 affinity between species, at different stages of development, and with some genetic diseases. Fetal hemoglobin (HbF) has a high affinity for O2, which is important for O2 transport across the placenta in utero. In the first year after birth, HbF is gradually replaced by adult hemoglobin, which has a lower affinity for O2. Hemoglobin S (HbS) results from a single amino acid substitution on the beta chain in sickle-cell anemia. Deoxygenated HbS tends to crystallize within red blood cells, distorting them into a crescent or sickle shape. Sickle cells are more fragile and less flexible than normal erythrocytes, and they tend to plug capillaries.

The three-dimensional shape of a hemoglobin molecule, which is determined by the allosteric interactions of its four subunits, causes the O2-equilibrium curve to be S shaped, or sigmoidal (Fig. 1). O2-equilibrium curves for individual alpha and beta chains are not sigmoidal, but simple convex curves like the O2-equilibrium curve for myoglobin. Myoglobin occurs in muscle and has only a single polypeptide chain with one heme group. The

Po2(mmHg)

FIGURE 1 Standard human blood-O2 equilibrium (or dissociation) curve at pH = 7.4, Pco2 = 40 mmHg, and 37°C. Left ordinate shows O2 saturation of hemoglobin available for O2 binding; right ordinate shows absolute O2 concentration in blood. Most O2 is bound to hemoglobin, and dissolved O2 contributes very little to total O2 concentration.

Po2(mmHg)

FIGURE 1 Standard human blood-O2 equilibrium (or dissociation) curve at pH = 7.4, Pco2 = 40 mmHg, and 37°C. Left ordinate shows O2 saturation of hemoglobin available for O2 binding; right ordinate shows absolute O2 concentration in blood. Most O2 is bound to hemoglobin, and dissolved O2 contributes very little to total O2 concentration.

sigmoidal shape of the O2-Hb equilibrium curve facilitates O2 loading on blood in the lungs, and O2 unloading from blood in the tissues, as explained in Chapter 21 on tissue gas exchange.

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

Get My Free Ebook


Post a comment