Describe the mechanisms used by mussels to obtain energy during the tidal cycle. Use these terms: High tide, low tide, short-term hypoxia, long-term hypoxia, aerobic metabolism, opine production, NAD+/NADH, PEP branch point, mitochondria, ATP yield, aspartate.
- during high tide, they are submerged and can extract oxygen from the water
- during low tide, the gills collapse so O2 cannot be efficiently extracted
- hypoxia and then anoxia ensue. In short term (<3 hr):
- pyruvate is converted into opines using 1 of 3 amino acids in a condensation reaction (restores NADH to NAD+ just like converting to lactate does)
- no transporters to remove opines from cells, unlike with lactate, lactate that excreted out of the cells would be lost to the environment in organisms with open circulatory systems
- this would result in a loss of the energy stored in the bonds of lactate
- Opine production maintains a lower [NADH] / [NAD+], which favors fermentation at the rate needed by invertebrate metabolism
- Opine production is better for invertebrate cells with lower metabolic demands
- during long-term hypoxia, mussels must prevent excessive acidification
- produce succinate and propionate instead of opines
- the PEP branch point continues RedOx reactions in mitochondria, consuming H+ and regenerating NAD+ BUT producing ATP slower than in the opine pathway
- acidification inhibits PK and activates PEP-CK, the enzyme used in the PEP pathway
- PEP has a higher ATP yield but takes longer, giving slower ATP production rates
- [ASPARTATE???]
What are two advantages of opine production over lactate production?
- Opines are retained in the cell in which they are produced (would be lost via excretion in an organism with an open circulatory system, resulting in a loss of an energy source)
- Opine production maintains a lower [NADH] / [NAD+], which favors fermentation at the rate needed by invertebrate metabolism
What triggers the activation of PEP-CK at the PEP branch point?
- acidification inhibits PK and activates PEP-CK, the enzyme used in the PEP pathway
- the higher concentration of H+ ions triggers the pathway intended to reduce the concentration
Define environmental and functional hypoxia, and provide examples
- environmental hypoxia results from low O2 in the environment and is mainly found in organisms with limited mobility that prevents them from actively escaping periods of environmental hypoxia
- EX: crucian carp, painted turtle, vampire squid, Amazonian fishes, intertidal mussels
- functional hypoxia is caused by burst-type energy in which the rate of ATP use is too high for the rate at which oxygen is being supplied and the cells must do anaerobic respiration despite plentiful oxygen in the environment
Why is acid-base regulation so important?
Acid-base homeostasis is essential for proper enzyme function (and therefore cell function and life)
- metabolic and physiological functions produce/consume H+
- environmental conditions can impact acid-base regulation as well
What are some of the implications of the pH scale being logarithmic?
pH change of 0.3 = 2-fold change in [H+]
pH change of 1 = 10-fold change in [H+]
How does pH affect enzyme function and activity?
- protein shape is determined by the interaction between acidic and basic amino acids
- pH affects the charge of the amino acids, affecting how they interact with each other => change in shape => altered function
- pH might affect protein structure, binding and catalytic sites
- Substrates and co-factors will not bind as efficiently
- Redox reactions will not happen as efficiently
Provide examples from the energy metabolic pathways we studied in class of pH affecting enzyme function (e.g. glycolysis, PEP branch point, hemoglobin)
- at the PEP branch point, the PEP-CK enzyme that triggers the alternative pathway only works in more acidic conditions while the PK (pyruvate kinase) enzyme, which is necessary in making pyruvate and following the lactate/opine pathways, cannot function in these more acidic conditions
- High CO2 and low pH reduce hemoglobin’s affinity for O2, leading to a right shift and the offloading of oxygen. This allows for oxygen to be deposited at the tissues, where there is a lot of CO2, and taken up at the lungs, where there is very little CO2
- Excessive intracellular acidification can inhibit core glycolysis and thus ATP production. Organisms must have pathways to deal with this acidification (EX: painted turtle and its shell mechanism) or they will die
- NKA has an optimum pH of 7.5
What are the different mechanisms for intracellular and extracellular acid-base regulation?
- passive mechanisms: buffering, does not require ATP, “Soak up” or donate H+
- active mechanisms: Use ATP to actively excrete and absorb H+and HCO3-
What is “buffering capacity”?
how much H+(or OH-) needs to be added to a system to change its pH by 1 unit.It depends on concentration and types of buffers present in the system
What are two characteristic of a good buffer system?
a good buffering system should:
- have PKa at the pH of the solution needs to be buffered
- consist of multiple buffers with different PKas.
What is the advantage of having multiple buffers with different pKa’s?
- buffering powers are additive
- will allow the solution to be buffered over a wide range of pHs
Explain the relationship between intracellular buffering capacity and muscle activity
- white muscle, which primarily does anaerobic respiration to power its burst-type energy, has a far higher buffering capacity to deal with the products of anaerobic respiration and prevent acidification
- red muscle, used for cruising, has lots of mitochondria and primarily does aerobic respiration. It has much lower buffering capacity
- high performance fish need a higher buffering capacity and have more white muscle than red muscle
Consider the white muscle of two different fish species. “Fish A” has 100 units of LDH activity, and the concentration of histidine compounds is 10 uM. “Fish B” has 2,000 units of LDH activity, and the concentration of histidine compounds is 200 uM. Which of the two fish species has a more active life style, “A” or “B”? Why?
- LDH activity describes the enzyme converting lactate into pyruvate
- fish B has more LDH activity, indicating that it is restoring its NAD+ more to carry out more fermentation, indicating a higher need for energy
- it also has more histidine compounds, which help to buffer acidic solutions (created by larger amounts of anaerobic respiration)
