648
CHAPTER 28
Hemoglobin
HOOC
T y r p i4 5 ( H C 2 ) ; r e s id u e
th a t lie s b e tw e e n h e lic e s
F a n d H in d e o x y -H b
P ro x im a l h is tid in e ; P 9 2 (F 8 ).
b o n d e d to fifth p o s itio n
th e h e m e iro n
H E M E G R O U P
C 2 T Q 7
O b in d in g site
D istal h is tid in e ; p 6 3 (E 7 ),
lo c a te d n e a r (b u t n o t
a tta c h e d to ) s ix th p o s itio
o f th e h e m e iron
F IG U R E 2 8 -3
Secondary structure of the
f)
chain of human hemoglobin. The helical regions (labeled A-H, after Kendrew), N and C
termini, and the histidines located near the heme group are indicated. The axes of the B, C, and D helices are indicated
by dashed lines.
tissues, the dissociation curve for myoglobin lies to the
left of the hemoglobin curves; consequently, hemoglobin
(Hb) can readily oxygenate myoglobin (Mb). In the
equation
Hb(0
2)4
+ 4Mb «=♦ Hb + 4M b0
2
the equilibrium lies far to the right.
In the hemoglobin curves, the change from A to B to
C is termed a rightward shift. The farther to the right a
curve is shifted, the larger is the P
50
value and the lower
is the oxygen affinity. In this example, the shift is due to
an increase in PCo2, since pH, temperature, and 2,3-DPG
concentration were held constant. However, the shift could
also have been caused by an increase in temperature (as
in strenuous exercise or fever), a decrease in pH (as in
acidosis or exercise), an increase in 2,3-DPG concentra-
tion (see below), or a combination of these variables. For
example, an increase in Pco
2
is usually accompanied by a
decrease in pH. Substances that cause a rightward shift are
called
negative allosteric effectorsov allosteric inhibitors
T A B L E 28-1
Partial Pressures of Oxygen
P02 (mm Hg)
Stage*
158
Inspired air
100
Alveolar air
95
Arterial blood
40
Peripheral capillary beds
<30 (est.)
Interstitial fluid
<10
(est.)
Inside cells in tissues
<2
(est.)
Inside mitochondria
*During transport from lungs to mitochondria in tissues, where 0 2 is
used as a terminal electron acceptor.
