What lowers pH in Bohr effect

[drXanthine]

In tissues that are exercising strenuously, the pH is lowered which helps hemoglobin unload O2 to the tissue via the Bohr effect. My question is what is the most significant contributor to the lowering of pH. I know that lactic acid is produced, which lowers pH. However, the tissues are also producing more CO2 which is converted into carbonic acid--also donating a proton. Do both factors contribute significantly to the lowering of pH?

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[jetsfan1234]

The CO2 is the major player here.

 

[Intraleukin-6]

In tissues that are exercising strenuously, the pH is lowered which helps hemoglobin unload O2 to the tissue via the Bohr effect. My question is what is the most significant contributor to the lowering of pH. I know that lactic acid is produced, which lowers pH. However, the tissues are also producing more CO2 which is converted into carbonic acid--also donating a proton. Do both factors contribute significantly to the lowering of pH?

I'm not sure if the tissues are producing more CO2. If a pathways are geared toward lactate production, then the pathways that produce CO2 aren't being utilized. Lactic Acid itself doesn't actually contribute to the drop in pH, but by hydrolysis of ATP. Lactate accepts the Hydrogen ion -> Lactic Acid. But that's kind of beyond the scope.

H + HCO3 <--> H2CO3 <--> H2O + CO2

I wouldn't assume CO2 would be converted into carbonic acid during exercise as much as the reverse is happening. High CO2 and low pH produce hyperventilation, shifting reaction towards the right. Low pH inhibits muscular contractions, so eventually things will shift toward the right. I would say CO2 has a net effect of alleviating an acidic state during exercise, but not in diseases which cause poor ventilation. It's all relative really.

 

[FeinMS]

I'm not sure if the tissues are producing more CO2. If a pathways are geared toward lactate production, then the pathways that produce CO2 aren't being utilized. Lactic Acid itself doesn't actually contribute to the drop in pH, but by hydrolysis of ATP. Lactate accepts the Hydrogen ion -> Lactic Acid. But that's kind of beyond the scope.

H + HCO3 <--> H2CO3 <--> H2O + CO2

I wouldn't assume CO2 would be converted into carbonic acid during exercise as much as the reverse is happening. High CO2 and low pH produce hyperventilation, shifting reaction towards the right. Low pH inhibits muscular contractions, so eventually things will shift toward the right. I would say CO2 has a net effect of alleviating an acidic state during exercise, but not in diseases which cause poor ventilation. It's all relative really.

I think the bolded statement is too extreme. While it's true that fermentation process will be running much more than the respiration process, the latter will not be completely turned off. When we exercise, we keep breathing in O2 and breathing out CO2. If the respiration process is completely turned off, where would O2 go and CO2 come from? Correct me if I'm wrong.

Also, I have a hard time why lactic acid will not have an effect on lowering pH. Lactic acid is formed at pH near 7 which is obviously above pKa of the lactic acid, so more lactate will be present that lactic acid. This process obviously release protons.

Also, while it's true that CO2+H2O <=> H2CO3 is happening at the same time during exercising, it occurs in different parts of the body. Conversion from CO2 to carbonic acid to bicarnonate occurs near tissue. This lower pH in tissues and lowers oxygen affinity of hemoglobins near tissues.
Bicarbonate travels to the lung where it's converted back to CO2 for exhalation. Since this process involves picking up a proton to make carbonic acid, pH near lung is higher than the normal pH. This increases oxygen affinity of hemoglobins near lung.

 

[jetsfan1234]

Tissues are ABSOLUTELY producing more CO2 during exercise. Aerobic respiration and all that. This is what causes the pH decrease --> Bohr effect.

 

[Intraleukin-6]

I think the bolded statement is too extreme. While it's true that fermentation process will be running much more than the respiration process, the latter will not be completely turned off. When we exercise, we keep breathing in O2 and breathing out CO2. If the respiration process is completely turned off, where would O2 go and CO2 come from? Correct me if I'm wrong.

I was talking more from a pathway standpoint. If a cell is producing lactate, then it probably isn't going to be producing CO2 from krebs. Once O2 is present aerobic respiration would continue.

Also, I have a hard time why lactic acid will not have an effect on lowering pH. Lactic acid is formed at pH near 7 which is obviously above pKa of the lactic acid, so more lactate will be present that lactic acid. This process obviously release protons.

True, I meant pyruvate is reduced to lactate lol. This consumes a proton as well as NADH.

Also, while it's true that CO2+H2O <=> H2CO3 is happening at the same time during exercising, it occurs in different parts of the body. Conversion from CO2 to carbonic acid to bicarnonate occurs near tissue. This lower pH in tissues and lowers oxygen affinity of hemoglobins near tissues.

But overall, there is a net shift toward CO2 conversion. A substantial amount of H+ ions aren't being formed because pH isn't changing very much, at least when compared to the increases in CO2 output. If that isn't convincing, look up respiratory exchange ratio. RER=Volume of CO2/Volume O2 with, and 1 is the max value at the cellular level (maximum carbohydrate consumption possible). When measured in a lab, RER regularly goes to 1.1-1.2 because of buffering and excess CO2 being expelled.