BIOC 450 10/31: Bioenergetics
- Catabolism refers to degradation of compounds, which produces energy and often involves convergence of many pathways into fewer (since the compounds are being broken down into increasingly elementary components.
- Anabolism refers to production of compounds, which requires energy and is often divergent (as a diverse range of products need to be synthesized from a few building blocks).
Basically, we want to capture free energy to produce useful things. Take the equation below:
aA + bB ⇌ cC ⇌ dD.
Keq = ([C]^c [D]^d)/([A]^a [B]^b) and ΔG°= -RT lnKeq.
Note that these are under biological conditions, so 7 pH, [H2O] = 55M, 1M initial concentrations, 1 atm, 25 °C. The logical meaning is that we start at 1M of each reactant/product, and we see if the reaction goes forward or back. If it goes forward, ΔG° is negative. We adjust this for the actual conditions, with the equation
ΔG = ΔG° + RTln(Q) where Q is Keq but with the real concentrations.
So generally, since [ATP] >> [ADP] and [Pi], ΔG for ATP hydrolysis is lower than ΔG°. (ΔG° = -30.5 kJ/mol). In an erythrocyte, it's more like -52 kJ/mol.
Cells use this — ADP levels are modulated to promote either favorable ATP synthesis (higher ΔG better) or ATP use (more negative ΔG better). Free energy changes are additive. We can couple ATP and an endergonic reaction to make it favorable. The P on the end of ATP is what gets moved around. P is added in phosphorylation by kinases. Certain amino acids can be phosphorylated, as well as lipids, etc.
ATP is a lot like GTP. It's an evolutionary accident that we only use ATP.
ATP releases so much free energy because its phosphates really want to leave, for a number of reasons:
- Electrostatic repulsion (between the oxygens)
- Greater solvation once it has left
- Increased entropy
- A separate phosphate can be resonance stabilized.
We can break the phosphate chain at any point, but only the gamma phosphate (the one on the end) is used mostly, because it is the most convenient.
If ATP's ΔG is so negative, why isn't it always losing its phosphates? It still has a high activation batter. We need an enzyme. Many enzymes use magnesium to stabilize the negative charge of the gamma phosphate. This draws electrons away from the phosphate bond, making it easier to break. Likewise, hydrolysis of other phosphoanhydrides and phosphoesters is generally favorable.
ΔG° of phosphate groups in various contests (phosphorylation potential) tells us whether a compound will be phosphorylated by ATP.