Enzymes

Enzymes are proteins that act as catalysts for reactions, such as those in cellular respiration. They increase the likelihood of a reaction by lowering the energy required for a reaction to occur.  In the process, enzymes are left unchanged by the reaction.  They may stress bonds in a molecule and increase the chance of that bond breaking and they may force molecules or atoms together and help in the formation of a new bond.  The molecules that bind to the enzyme are called substrates.  The resulting molecules from the reaction are called products. 

 

Enzymes may have more than one binding site. 

Reactions take place when substrates fit into the active site of an enzyme.

Enzyme activity may also be regulated by molecules binding to other sites called regulatory sites or some inhibitors may block substrates from binding to an active site. 

Helpful Links on Enzymes

Enzyme animation 1

Enzyme animation 2 [Click on "Biochemical Pathway"]

Enzyme activity

 

Cellular respiration is the process whereby energy is harvested from carbon compounds (food molecules) as they pass through a series of enzyme catalyzed reactions.  The enzymatic reactions of cellular respiration begin in the cytoplasm, but most of the reactions occur in the mitochondria. 

Mitochondrion Structural Overview

Cellular respiration occurs in the double-membrane organelle called the mitochondrion.  The folds in the inner membrane are called cristae.
 

 
Overview of cellular respiration:

·         Glucose passes through a series of catabolic reactions in the process of cellular respiration.  First, glucose is split in the cytoplasm in the process of glycolysis.   The resulting end-products move into the mitochondria and go through a preparatory step to the Krebs cycle, and finally to the electron transport system.   

 

·         The useful energy in a glucose molecule is stored in the covalent bonds, which are formed when atoms share electrons.  Thus the energy is in the electrons.  These high-energy electrons are removed from the glucose molecule, added to carrier molecules and are transported to the electron transport system.  Their energy is used to form the high-energy bonds of ATP and the spent electrons are added to oxygen to make water.


Electron Carriers:

 

Glycolysis:

 

Preparatory Step

  • Occurs in mitochondrial matrix

  • 3-carbon pyruvate enters and a CO2 is removed leaving a 2-carbon acetyl group.

  • 2-carbon acetyl group binds to CoenzymeA producing acetyl-CoA that goes to the Krebs cycle

 

Krebs Cycle (also called the Citric acid or Tricarboxylic acid cycle)

 

  • Also occurs in matrix of mitochondria

  • Acetly-CoA enters dropping off the 2-carbon acetyl group which is added to a 4 carbon group.  The CoA returns to the preparatory step to get another 2-carbon acetyl group

  • During this series of reactions, high-energy electrons are removed and transferred to empty carriers (NAD+ and FAD)

  • These full electron carriers, NADH and FADH2 enter the ETS

  • During these reactions the two carbons from the acetyl group are eventually removed as carbon dioxide

  • 1 ATP is made for each acetyl-CoA that enters the Krebs cycle

 

Preparatory Step and Krebs Cycle Animation

 

 

For one glucose molecule that has moved through glycolysis, the preparatory step and the Krebs cycle, answer the following questions:

1.      How many ATP have been generated so far? ______________

2.      How many NADH have been formed?

3.      How many FADH2 have been formed?

4.      How many CO2 molecules have been produced?

5.      Has any O2 been used?

 

Electrons from glycolysis, the prep-step and the Krebs cycle are carried in NADH and FADH2 to the ETS.

Electron Transport System (ETS) or Chain (ETC)
  • The ETS is a series of electron carriers on the cristae of inner membrane of mitochondria
  • The NADH and FADH2 bind to proteins in the ETS and the electrons that they are carrying are transferred to the ETS.  The protons are released as H+.
  • Proton pumps (using the energy from the electrons) move the H+ from the matrix to intermembrane space, creating a high concentration of H+ in the intermembrane space
  • Electrons lose their energy as they move down the ETS (driving the H+ pumps).
  • At the end of the ETS, two low-energy electrons along with two H+ bind to oxygen (1/2 of an O2 molecule), the final electron acceptor, forming H2O.

 

ATP Synthesis [Finally J]:

  • There is a higher concentration of H+ in intermembrane space of mitochondria than in the matrix.
  • H+ flows down its concentration gradient toward the matrix providing energy.
  • H+ moves through the ATP synthase enzyme.  As H+ moves down its gradient from the intermembrane space to the matrix, ATP synthase (in cristae) adds phosphate to ADP producing ATP (ADP + P --> ATP).

ETS Animation