分子与细胞生物学 2 - L2abc Reactions and Enzymes (1)

酵的内容分为两部分：第一部分是基本的反应以及耦合机制，第二部分是与酵相关的实验室技术。结构机制会放在在蛋白质的部分(所以说暂时看不到封面图哈)~ 不太明白的或者有错误的地方随时来找UP主喔~ 本部分的参考文献Essential Cell Biology, 5th ed. Alberts, et al. 2019. 部分内容来自khanacademy与维基百科.

封面图 https://nayadaur.tv/wp-content/uploads/2019/05/Enzymes.png

2 Reactions and Enzymes (1)

2a Introduction

Molecule changes by doing chemical reactions. Cells use enzymes to promote chemical reactions on desire.

Free energy (G)

The total energy in a molecule - stored in bonds + various types of movements

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| 注意绝对值，这是个坑··· 别问，问就是被扣过不少分

Example:

Photosynthesis - uphill, sugar - CO2

Respiration - downhill, CO2 - sugar

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

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!

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!

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

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 (这个数字不用记，用来跟下面的比较)

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

Example:

Phosphorylated molecules are activated carriers - store energy (ATP to ADP) used to drive uphill synthesis reactions.

Hydrolysis reaction (左下图) - water is used to split a molecule

Always favorable, downhill reaction

Reverse: Condensation reaction (右上图)

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

下期2d: Lab techniques (超短警告：话说下期这么短为什么要跟这期分开呢？别问，问就是想多水一期哈哈哈~)