Page 80 - Basic Monitoring in Canine and Feline Emergency Patients

(A) (B)
CH
C H 2 2 CH 3 3
C
H
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H 3 C C
H 3
N N NN
C
CH 2
c
β chain 1 β β chain 2 2
h
n
i
a
Fe
Fe
N N N N
C
H 3 3 C CH 3
H C
H
2+
Fe
Fe 2+
Heme
Heme
Heme
He m e
α chain 2
α chain 1 α c h a i n 2
COOH COOH
COOH
COO
H
Fig. 4.1. A diagram of the structure of (A) hemoglobin and (B) heme. Note the two alpha and two beta globin chains,
each with their associated heme subunits for oxygen binding. Open source Illustration from Anatomy & Physiology,
Connexions https://commons.wikimedia.org/wiki/File:1904_Hemoglobin.jpg. (accessed 3 August 2019).
at the tissues. Oxygen can only bind to Hb in the
3+
ferrous (Fe ) state; an irreversible change to Fe
2+
NHNH N N would render the molecule useless for carrying
oxygen and the Hb would become non-functional
as an O -transport protein. Therefore, to allow
HN
N N HN 2
2+
the Fe –O binding to be reversible in vivo, the
2
globin portion of Hb contains approximately 20
amino acid residues that help to stabilize this
Fig. 4.2. The chemical structure of a porphyrin ring. interaction.
Several varieties of porphyrin rings exist and are The most important of these amino acids is
capable of binding with different metal ions. Porphyrins histidine which donates a negative charge to sta-
containing iron molecules are called heme. Open bilize the Fe –O complex, changing the confor-
2+
2
source illustration from https://commons.wikimedia.org/ mation of the surrounding globin proteins beyond
wiki/File:Porphyrin.svg (accessed 3 August 2019).
just that single binding site and also affecting the
other three binding sites within the same Hb com-
cyanotic animals can be presumed severely hypox- plex. This protein conformation with one O
2
emic without further testing necessary (e.g. ‘blue is molecule binding to Fe is termed a ‘relaxed
2+
bad’), but the lack of cyanosis does not necessarily state’ (RHb). In this relaxed state, Hb’s affinity
indicate normal oxygen saturation (‘pink is not for oxygen increases by 150%, which facilitates
necessarily OK’). rapid binding of the other three heme sites to O
2
Normally in nature, when Fe reacts with O molecules. In contrast, when no oxygen is bound
2+
2
it produces ferric iron (Fe ) as an irreversible to any Fe , Hb undergoes a conformational
3+
2+
oxidation reaction. A simple example of this change which disrupts the porphyrin ring struc-
reaction is rust on a cast iron skillet. However, ture. This results in Hb entering a ‘tensed’ state
the interaction between the iron molecule in (THb). The structure of THb inhibits oxygen
heme and oxygen must be fully reversible under binding at any site; therefore, DeOxyHb has a
normal physiologic conditions to allow for easy low affinity for oxygen. This means that any
uploading of oxygen in the lungs and offloading given Hb protein in the blood prefers to be either
72 K.A. Marshall and A.C. Brooks