Enzyme Browning Reaction





Phenolase exist in common daily activates since they are found on a variety of fruits and vegetables that are consumed. When the fruits and vegetables such as mushrooms, potatoes and bananas are damaged it tends to turn brown over time because the enzyme phenolase present in them are exposed to oxygen and hence oxidizes the substrate found in the cells. 

Enzyme browning reaction can be controlled by several procedures such as the inactivation of the enzyme and the removal of oxygen. A process called blanching is a very effective way in controlling enzyme browning reaction by heating the fruits or vegetable to denature the enzyme before storing. In addition to heating the fruit or vegetable can be stored in ice to prevent oxidation.  Also the addition of citric acid like those found in lime or vinegar decreases the pH at which the enzyme would normally function hence inhibiting enzymatic activity.


Test Your Knowledge ……. TCA….


1. Name the enzyme found in the Kreb Cycle (TCA) that is similar to pyruvate dehydrogenase.

(A) Alpha-ketoglutarate Dehydrogenase

(B) NADH Dehydrogenase

(C) Coenzyme A

(D) Malate Dehydrogenase

(E) Fumarase


What is the correct order os step 1 in the Kreb Cycle

1. S-citryl CoA undergoes hydrolysis to from citrate

2. An enzyme removes a proton from the CH3 in CoA

3. This causes a strong forward reaction.

4. The negatively charged CH2 bond to the oxaloacetate carbon


(A) 1, 2, 3, 4

(B) 2, 1, 3, 4

(C) 3,1,4, 2,

(D) 2, 4, 1, 3

(E) 4, 2, 3, 1


Ah Riddle Ah Riddle Ah Ree… ETC Quiz


1. Why does ATP synthase undergo conformational changes in the electron transport chain?

1. To generate ATP from ADP + P

2. To move protons from the inter-membrane to the matrix of the mitochondria

3. To move protons from the inter-membrane to the outer membrane

4. To generate ADP + P using ATP


(A) 1 only

(B) 1, 2,

(C) 2, 3

(D) 2, 4

(E) All of the above


2. Rotenone is an inhibitor that disrupts the electron transport chain complexes. Which of the complex is disrupted?

(A) Complex 1

(B) Complex 2

(C) Complex 3

(D) Complex 4

(E) ATP Synthase Complex


Inhibitors of ETC

Cyanide poison inhibits complex 2, cytochrome oxidase, of the electron transport chain. It binds to the Fe+++ in the hem group in complex 2 interfering with the pumping of the protons and the flow of reaction. Therefore a electrochemical gradient would not be creating hence ATP synthase would not release the ATP for the ions to move into the matrix.

Rotenone acts much like cyanide, however it inhibits complex 1. NADP Dehydrogenase.

Oligomycin  inhibits mitochondrial H+-ATP synthase by binding to the Oligomycin sensitivity-conferring protein (OSCP) at the F(o) subunits 6 and 9  which are found in the stalk of the F1F0-ATPase complex. This binding blocks the proton conductance and inhibits the synthesis of mitochondrial ATP. Therefore the electrochemical gradient would increase in the intermembrane and decrease in the matrix until the protons runs out.

2, 4 DNP is known as an uncouple as it only affects one complex. It is hydrophobic and has the ability to release protons from the inter-membrane space to the matrix by perforating the wall on the inner membrane while it is lodge in it. Therefore no energy is require for the active transport and the energy made is released tot he surroundings as heat. The ETC still works.

TCA – Link Reaction



  • Two pyruvates are formed from glycolysis in the cytoplasm.
  • Most of the energy that was stored in the glucose molecule is still present in pyruvic acid.
  • When oxygen is present, pyruvate is transported to the mitochondrial matrix and more reactions take place, freeing more energy.
  • The link reaction involves oxidative decarboxylation of pyruvate: oxidative because an H+ is removed; decarboxylation because a CO2 is removed.
  • And the link reaction involves the reduction of NAD+ to NADH, and it produces acetyl coenzyme A (acetyl CoA) and CO2.
  • Net yield of 2reduced NADH per glucose
  • Pyruvate Dehydrogenase Cofactors

    • TPP: thiazolium ring adds to  carbon of pyruvate, then stabilizes the resulting carbanion by acting as an electron sink.
    •  Lipoic acid: oxidizes pyruvate to level of acetate (acetylCoA), and activates acetate as a thioester.
    • CoA-SH: activates acetate as thioester.
    • FAD: oxidizes lipoic acid.
    • NAD+ : oxidizes FAD.

The Kreb Cycle Made Easy

So i always despised the Kreb cycle.. soo long and tedious all those reactions and enzymes… sigh … -_-

That is until i found this……: an awesome.. description with explanatory diagrams


The Entire Kreb Cycle 

Capture 1

The Kreb Cycle Broken into Individual Reactions

Capture 2 Capture 3 Capture 4













So———-citrate Synthetase



Dance———-isocitrate Dehydrogenase

Devon———-alpha-ketoglutarate Dehydrogenase

Sipped———-Succinyl-CoA Synthetase

Down———-Succinate Dehydrogenase


Drinks———-malate Dehydrogenase


GLYCOLYSIS — Quiz time


1.  Which of the enzyme(s) are irreversible in the prepatory phase?

1..Phosphohexose Isomerase
2. Hexokinase
3. Aldose
4. Phosohofructo-kinase 1

(A) 1 only

(B)  1, 2

(C)  1, 3

(D)  2, 4

(E)   3, 4

2. In glycolysis process, the step(s) that produces ATP is called…

(A)  First priming reaction

(B)  Oxidative phosphorylation

(C)  Substrate level phosphorylation

(D)  Oxidation

(E)  Reductive phosphorylation


Lipids can be broken up into  triglycerides (fats and oil) and waxes, steroids, terpenes and phospholipid.

Properties of Lipid

  • insoluble in water
  • soluble in non-polar solvent such as benzene
  • they contain carbon, hydrogen and oxygen but less oxygen but more hydrogen than carbohydrates
  • they generally give twice as much energy when they respire than carbohydrates
  • less dense than water

The most common types are triglycerides usually known as fats and oils. Fats are solid at room temperature and oils are liquid. Triglycerides are made by the combination of three fatty acids with one glycerol. The higher the proportion of unsaturated fatty acids the more likely they will be liquid at room temperature. Each of the fatty acids joins to glycerol by condensation reaction.

Triglycerides are n on-polar. This means there is no uneven distribution of charge within the molecule so they cannot form hydrogen bonds with water and therefore do not dissolve in water. They are soluble in organic solvents such as ether and chloroform. They are hydrophobic and are less dense than water.


Structure of Glycerol


Fatty acids contain an acidic group –COOH. The general formula is R.COOH where R is hydrogen or a group such as –CH2, C2H5 increasing by CH2 for each subsequent member. The long chain of carbon and hydrogen atoms forms a hydrocarbon tail. The tail is hydrophobic and non-polar making the triglyceride insoluble in water.

Fatty acid sometimes contains one or more double bonds. If it does, the fatty acid, as well as the lipid containing them are said to be unsaturated. Fatty acids and lipids lacking double bonds are described as saturated. Double bonds make fatty acids and lipids melt more easily, so most oils are unsaturated.


Formation of a Triglyceride




The Role of Triglycerides

  • Energy stores – a given mass of lipid will yield more energy on oxidation than an equal mass of carbohydrate. This is because lipids have a higher proportion of hydrogen atoms and an almost insignificant proportion of oxygen as compared with carbohydrates
  • Animal store extra fat when hibernating and fat is also found below the dermis of the skin of vertebrates where it serves as an insulator. It is extensive in mammals in cold climates, particularly in the form of blubber in aquatic mammals such as whales and also contributes to its buoyancy.
  • It is a metabolic source of water. When oxidized in respiration, it forms carbon dioxide and water. This is important in some desert animals such as kangaroo rat, which never drinks water and survives on metabolic water from its fat intake.

Waxes are lipid molecules whose alcohol is not glycerol and they usually have 1 hydrogen chain e.g. chitin 

Steroids  are ring compounds e.g. Vitamin D, sex hormones such as testosterone, 

Terpenes are similar to steroids but smaller e.g vitamin A, C, K


In a phospholipid, one of the three fatty molecules is replaced by a phosphate group, which carries an electrical charge and can therefore dissolve in water. the phosphate group is hydrophillic.  These have a polar phosphate head and two non-fatty acid tails. Polar groups or molecules are charged and have an affinity for water (hydrophillic). Non- polar groups or fatty acids do  not mix with water (hydrophobic). The phospholipids arrange themselves as a bilayer with the hydrophillic head facing the aqueous external and the internal environment and the hydrophobic tails shielded forming a micelle. Some of the phospholipod tails are saturated and some are unsaturated. The more unsaturated they are , the more fluid the membrane. This is because the unsaturated tails are bent or have kinks which prevent close packing so they fit together loosely. The shorter the length of the fatty acid tails, the more fluid the membrane

Membranes phospholipids move within the membrane by oscillation of the fatty acid tails (oscillation being greatest away from the head), flip-flop (less often) and lateral diffusion.



These are phospholipids with short branching carbohydrate chains attached to the external surface. They also polar heads and non-polar tail.


is completely and thus found in the hydrophobic region of the phospholipid bilayer. Cholesterol regulates membrane fluidity preventing it from becoming too fluid or to rigid. Fluidity is advantageous as it allows the membrane to seal itself. Cholesterol provides mechanical stability as without membranes quickly break and cells burst open. Cholesterol also reduces uncontrolled leakage, by diffusion, of polar molecules, ions and water through the membrane ensuring that they must pass through special channels where they are controlled.