分子與細胞生物學 2 - L2abc Reactions and Enzymes (1)
酵的內(nèi)容分為兩部分：第一部分是基本的反應以及耦合機制，第二部分是與酵相關(guān)的實驗室技術(shù)。結(jié)構(gòu)機制會放在在蛋白質(zhì)的部分(所以說暫時看不到封面圖哈)~?不太明白的或者有錯誤的地方隨時來找UP主喔~ 本部分的參考文獻Essential Cell Biology, 5th ed. Alberts, et al. 2019. 部分內(nèi)容來自khanacademy與維基百科.
封面圖 https://nayadaur.tv/wp-content/uploads/2019/05/Enzymes.png
2 Reactions and Enzymes (1)
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2a Introduction
Molecule changes by doing chemical reactions. Cells use enzymes to promote chemical reactions on desire.
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Free energy (G)
The total energy in a molecule - stored in bonds + various types of movements
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Free energy changes during a reaction
Y axis: free energy
X axis: reactant - product
ΔG = Gp - Gr
Gp > Gr, Slope > 0, uphill reaction, unfavorable, requires energy input
Gp < Gr, Slope < 0, downhill reaction, favorable, release energy
Amount of free-energy change = |ΔG| 注意絕對值，這是個坑··· 別問，問就是被扣過不少分
Example:
Photosynthesis - uphill, sugar - CO2
Respiration - downhill, CO2?- sugar
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Sugar to CO2
Favorable, but why not decompose in the kitchen??Barrier!
Activation barrier
Downhill reaction does not occur rapidly, a uphill is needed before the downhill. Uphill reactions also have activation barrier, it goes higher and take the downhill after
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ΔG determines whether it is a overall favorable reaction
Activation barrier determines how fast the reaction proceed
Sugar breakdown - need to be?on fire?first!
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Two ways to get over the barrier
Add energy - heat the reaction
- Uncatalyzed reaction - energy not large enough to surmount barrier
Lower the barrier - add the enzyme
Lower the activation barrier
Increase rate of reaction
Do not change ΔG!
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Enzyme
Heat accelerates reactions non-specifically; enzymes only accelerate specific reactions
Can be in the normal temperature
Can drive unfavorable reactions by coupling them to favorable reactions - the enzyme connects two reactions - energy released by one downhill reaction drives the other uphill reaction.
- Two coupling mechanisms: Sequential reactions & Activated carriers
2b Enzyme-coupled reactions: Sequential reactions
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Accelerating slow reactions: enzyme catalysis
Driving uphill reactions: enzyme-coupled reactions
Enzyme can drive unfavorable reactions by coupling them to favorable reactions
F here is synthesized from A by a series of sequential reactions (the product of one reaction is the reactant for the next)
Unfavorable reaction blocks the pathway?
No. A pathway can pass through an unfavorable step if the next step is favorable, so that the coupled sequential reactions can be favorable overall. So B-C can occur because it is coupled to C-D.
2c Enzyme-coupled reactions: Activated carriers
Activated carriers store energy (eg ATP?三磷酸腺苷) - bring it to wherever it’s needed. The energy will later be used to drive uphill reactions.
Many phosphorylated molecules release energy when hydrolyzed
ATP - ADP +Pi ? ?ΔG = -7.3 kcal/mole (這個數(shù)字不用記，用來跟下面的比較)
glucose-6-P - glucose +Pi ?ΔG = -3.3 kcal/mole
The hydrolysis of one molecule (blue) can be used to drive the phosphorylation of another (green).
Only works when the energy releases from the blue square is more than needed to add the Pi to the green triangle
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Example:
Phosphorylated molecules are activated carriers - store energy (ATP to ADP) used to drive uphill synthesis reactions.
Hydrolysis reaction (左下圖) - water is used to split a molecule
Always favorable, downhill reaction
Reverse: Condensation reaction (右上圖)
Water is formed when chemical groups are joined - unfavorable
Biological polymers made from condensation reactions:
Sugar - polysaccharide?(starch, glycogen)
Amino acid - protein
Nucleotide - nucleic acid
Hydrolysis vs Condensation
下期2d: Lab techniques (超短警告：話說下期這么短為什么要跟這期分開呢？別問，問就是想多水一期哈哈哈~)
