Ch 7 Text 3
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· Ability to deliver oxygen to tissues is enhanced by __
· To release oxygen where the need is greatest, hemoglobin has evolved to respond to higher levels of these substances
· H+ and carbon dioxide are __ that bind to sites on hemoglobin __= __
the facility of hemoglobin to respond to cues that signal the need for oxygen, such as in rapidly metabolizing tissues
different from oxygen-binding sites= Bohr effect
Explain the relationship between hemoglobin and pH.
The oxygen affinity of hemoglobin decreases as pH decreases from a value of 7.4à oxygen more likely to be released
Chemical basis of the Bohr effect
At least two sets of chemical groups are important for sensing changes in pH: the __ and __, which have pKa values near 7
Consider histidine B146. In deoxyhemoglobin, the __ forms a __ with a __ in the __ of the other __--> what does this lead to?
alpha-amino groups at the amino termini of the alpha chain and the side chains of histidines B146 and alpha-122
- terminal carboxylate group of B146
- salt bridge
- lysine residue
alpha subunit of the other alpha-beta dimer
locks the side chain of histidine B-146 in a position from which it can participate in a salt bridge with negatively charged aspartate b-94 in the same chain
The other groups also participate in __ in the __. The formation of these salt bridges does what?
Carbon dioxide, a __species, passes through the __ into the cell, helped by __associated with __
- salt bridges in the T state
- stabilizes the T state, leading to a greater tendency for oxygen to be released
- red blood cell membrane
- Rh blood types
o Carbon dioxide stimulates oxygen release by two mechanisms
What are they?
- § High concentrations of CO2 à drop in pH
- § A direct chemical interaction between carbon dioxide and hemoglobin stimulates oxygen release
§ High concentrations of CO2 à drop in pH
CO2 reacts with water to form H2CO3, which is accelerated by carbonic anhydrate. Once formed, H2CO3 dissociates to form bicarbonate ion and H+ --> drop in pH that stabilizes the T state
§ A direct chemical interaction between carbon dioxide and hemoglobin stimulates oxygen release
The effect of carbon dioxide on oxygen affinity can be seen by comparing oxygen-binding curves in the absence and in the presence of carbon dioxide at a constant pH.
What happens in the presence?
In the presence of carbon dioxide at a partial pressure of 40 torr at pH 7.2, the amount of oxygen released approaches 90% of the maximum carrying capacity. Carbon dioxide stabilizes deoxyhemoglobin by reacting with the terminal amino groups to form carbamate groups, which are negatively charged, in contrast with the neutral or positive charges on the free amino groups.
The amino termini lie at the interface between the __, and these __ participate in __ that stabilize the T state, favoring the __
- alpha-beta dimers
- negatively charged carbamate groups
- self-bridge interactions
- release of oxygen
Carbamate formation also provides a mechanism for __, but it accounts for only about 14% of the total carbon dioxide transport. Most carbon dioxide released from red blood cells is transported to the lungs in the form of__produced from the __inside cell
- carbon dioxide transport from tissues to the lungs
- hydration of carbon dioxide
that is formed leaves the cell through a __ that __. Thus, the serum concentration of HCO3-
__. By this means, a large concentration of carbon dioxide is transported from __ to the __ in the form of __.-
- specific membrane-transport protein
- exchanges HCO3- from one side of the membrane for Cl- form the other side.
- tissues to the lungs in the form of HCO3-
In the lungs, this process is reversed: how?
. Thus, carbon dioxide generated by active tissues contributes to a __and, hence, to __ and is converted into a form that can be transported in the __ and released in the __
- HCO3- is converted back into carbon dioxide and exhaled
- decrease in RBC pH
- oxygen release
- serum lungs
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