Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 79

Tabl e 3. 7. Functional and dysfunctional types of hemoglobin compounds. A single hemoglobin
molecule can transport 4 oxygen molecules. Each erythrocyte contains about 3 10 8 hemoglobin

molecules. The affinity of hemoglobin for oxygen depends on 4 primary regulators, CO 2 , hydrogen
ion (H + ), chloride ion (Cl − ), and (2,3)-bisphosphoglycerate (BPG). Hemoglobin can bind, in
particular, protons and CO 2 that cause a Hb conformational change and support 0 2 release. In lung
capillaries, H + and CO 2 are liberated from hemoglobin. Higher BPG concentrations in blood
enables the delivery of larger O 2 amounts to cells under smaller blood O 2 content (high altitude).
Normally 1 to 2% hemoglobin is methemoglobin. NADH-dependent methemoglobin reductase
converts methemoglobin to hemoglobin. Hemoglobin has a competitive binding affinity for cyanide
that contains the cyano group (-CN − ), sulfur monoxide (SO), nitrogen dioxide (NO 2 ), and sulfide
(S 2 − ), including hydrogen sulfide (H 2 S), that bind to iron (Fe 2 + ) in heme and prevent O 2 binding.
Cyanide is a natural, potent cellular toxin. Acute cyanide toxicity results from accidental exposure
or iatrogenic toxicity. Sulfide is a sulfur dianion in its lowest oxidation state. Hydrogen sulfide is a
very toxic gas and yet another natural messenger.

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Type Associated compound

Functional hemoglobin types

Carbaminohemoglobin Carbon dioxide

( CO 2 Hb) Lowers hemoglobin affinity for 0 2 (Bohr effect)

CO 2 + Hb NH 2 ←→ H + + Hb NH −− COO −

Oxyhemoglobin Oxygen

( O 2 Hb) Taut and relaxed forms have low and high

oxygen affinity, respectively

Glycated hemoglobin Glucose

Nitrosyl hemoglobin Nitric oxide

(Hb NO )
Nitrohemoglobin

Nitric oxide (S-nitrosylation)

or nitrosohemoglobin
(Hb SNO )

Dysfunctional hemoglobin types

Carboxyhemoglobin Carbon monoxide (inhaled or metabolic product)

( CO Hb) Stable complex

CN − anion (via Fe 2 + )
Unability to transport oxygen

Cyanohemoglobin

Methemoglobin Ferric rather than ferrous ion

Fe 3 + Hb

Cannot bind oxygen

Sulfur atom upon combination H 2 SandFe 3 + Sulfhemoglobin

( S Hb)

Usually caused by drugs

nitrous oxide (N 2 O) can convert a small fraction of hemoglobin to methemoglobin.
Normally, only 1 to 2% of hemoglobin is methemoglobin.
Other molecules compete to bind hemoglobin, such as cyanide (CN − ), sulfur
monoxide (SO), nitrogen dioxide (NO 2 ), sulfide (S 2 − ), and hydrogen sulfide (H 2 S).