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生命的化學工業：化學反應與酵素
10-12-2015
生物有別於非生物的特點
能量的吸收與利用 – 結構的維持與更新
Life can only occur in an open system
with continuous flux of energy and material.
Thermodynamics
• First law: energy is conserved.
不能利用的動能轉變成可以利用的位能
能量：生命運作的動力
能量的轉變
Energy transformation
Make all living organisms
on earth alive!
食物分子中生命不能利用的化學能,經由細胞的代謝反應
轉變成生命可以利用的化學能
Metabolic Pathways
How to judge whether one
chemical reaction will
spontaneously occur or not?
The road to equilibrium is down
the free energy hill!
But how to express it?
自由能的大小决定化學反應的方向及容易的程度！
Some remarks on the Gibbs free energy
• It's not necessarily "free“
– ΔG is the maximum amount of energy which can
be “freed” from the system to perform useful work.
• It's not energy!
– it is not conserved.
• It's not even "real"!
– it has no physical reality as a property of matter
– it is simply a useful construct that serves as a
criterion for change and makes calculations easier
Gibb’s free energy is a state function
that expresses the spontaneity of a
chemical reaction in terms of enthalpy
and entropy change of the system.
平衡常數可指出化學反應發生的方向，以葡萄糖氧化為例，Keq 可以
高達 10504 ，表示此反應到達平衡時，幾乎百分之百的葡萄糖都會
被氧化掉！
Thermodynamic aspect of
chemical reaction
Does the reaction occur?
At which direction?
Proceed to what extend?
Enthalpy and equilibrium constant
But it will not tell us how fast of the
reaction!
Kenetic aspect of chemical reaction
How fast (rate) of the reaction occur?
Zero, first and second order kinetics
Hydrogen + Oxygen = Water + heat
Explosion!
放熱反應：△G 為負值，反應能自動進行!
Hydrogen + Oxygen = Water + heat
Will it happen?
Never at room temperature!
Why?
You need match!
Hydrogen + Oxygen = Water + heat
Match provides a little bite of energy (activation energy) to break
the chemical bond between two hydrogen and oxygen atoms.
Then the oxygen and hydrogen atom join to form a new (stronger)
chemical bond.
More energy will be released as heat to the environment.
The total energy balance is positive.
Do we have such reaction in our body?
Can or how we utilize the heat released from this reaction?
hydrogen
hydrogen
How to make a chemical reaction occurring faster?
(活化能)
自由能
To keep temperature high; to put high pressure
or to increase concentration of the reactants; or
To reduce activation energy by catalyst
Catalyst can only accelerate reaction rate, but would
not change the net energy balance or equilibrium of the
chemical reaction.
Every chemical reaction in living system needs catalyst
to make reaction occurring possible at room
temperature and pressure!
The catalyst in living system is “enzyme”.
Most of enzymes are made of protein.
生命運作的主角 : 蛋白質
To make chemical reaction (synthesis and degradation) occur in the cell
04_09_Proteins.jpg
Enzyme binds specific molecules: substrate!
Just like lock and key ?
Yes and No!
04_31_specific ligands.jpg
Hexokinase Undergoes Induced Fit on
Substrate Binding
The rate accelerations caused by five different enzymes
How does enzyme accelerates reaction rate?
Turnover number of enzyme:一個enzyme分子每
秒可以轉變幾個分子的 substrate into products.
Ribozyme: RNA can be
an enzyme (catalysis)
Why RNA but not DNA has catalytic power?
How enzyme activity can be regulated?
• Amount of enzyme protein (gene expression)
–
–
–
–
Transcriptional rate
Stability of mRNA
Translational rate
Stability of protein
• Post translational modification
– Cleavage of proenzyme into active enzyme
– Chemical modification such as phosphorylation
• Environmental factors
– Temperature, pH, inhibitors, activators etc.
Which amino acid may participate in
pepsin’s catalysis?
Allosteric inhibitor or activator shares similar working mechanism.
Synthesize C, U and T
04_39_conform.change.jpg
By substrate concentration!
Glucokinase: Km = 10 mM,
Present in liver and in
pancreas b cells.
Hexokinase: Km= 0.2 mM,
Present in most cells.
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