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Text (7th ed.): Ch. 2, pp. 39-53 12/13/99

Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)

possible web site: http://esg-www.mit.edu:8001/esgbio/glycolysis/dir.html

(MIT Biology hypertext on these energy-generating pathways)

1. From last week, an enzyme is a protein catalyst (name ends in -ase). The chemicals that are involved in the chemical reaction that an enzyme affects are called substrates. These substrates fit within the active site (catalytic site) of the enzyme, forming an ezyme-substrate complex before forming the products.

2. How can one regulate enzyme activity in a controlled fashion? Some chemicals (activators) speed up an enzyme's rate and other chemicals (inhibitors) slow down the enzyme. An enzyme can have allosteric change, where these modifying chemicals bind to the enzyme and change its shape, and competitive inhibition, where inhibitors can occupy the active site instead of the substrate. Many of your"daily vitamins" are part of coenzymes that assist enzymes. The take-home lesson is that biological control of an enzyme is often through controlling the shape of the enzyme, and hence, its active site.

3. What supplies the energy to make all of this biological activity go? The organic molecule ATP (adenosine triphosphate) has three subunits to make it the currency of energy in animals and other organisms: an adenine or organic ring molecule, a ribose or sugar, and three phosphate groups connected in a linear fashion. The outermost two are connected by a high-energy bond that results from the repelling effects of the adjacent phosphate groups. This high energy bond is a property of the entire molecule, as the phosphate bonds themselves have a low activation energy, but give off a lot of energy when released, which drives the energy-requiring reactions in a cell.

ATP + H2O à ADP + Pi (inorganic phosphate) + energy.

4. ATP is constantly being used up and animal cells are constantly recycling the leftover ADP back to ATP by putting in energy from food sources. ATP can be made in basically two ways:

4A. One (substrate-level phosphorylation) couples ADP + Pi + energy à ATP + H2O with energy-producing reactions to produce the needed driving energy.

4B. The more common way (chemiosmosis) is the pumping of protons (i.e., hydrogen ions) across the inner mitochondrion membrane. A pool of hydrogen ions piles up in the outer compartment of the mitochondrion. We will see how these hydrogen ions are formed from the chemical bonds in food via a process called cellular or oxidative respiration.

Ultimately, when these hydrogen ions are driven through an ATPase (an enzyme) in the inner mitochondrial membrane, ATP is made from ADP and Pi.

5. As the name implies, oxidative respiration requires air in order to work. This is the example of an aerobic process. In contrast, an anaerobic process such as fermentation takes place without air. Both of these are used to break down the products of the process known as glycolysis.

6. Glycolysis is the initial sequence that breaks down a six-carbon sugar molecule (glucose) into two molecules of a three-carbon sugar (pyruvate) in the cytoplasm.

Glycolysis does not require oxygen, takes several separate chemical steps, and produces 2 ATP molecules per molecule of glucose.

One possible pathway after glycolysis and its end product pyruvate is fermentation, where:1

pyruvate + NADH à reduced compound + NAD+.

This allows for the produced NAD+ to be recycled into NADH, and besides, a cell has plenty of opportunity to break down the reduced compound later. In the case of tired muscles the reduced compound is lactic acid. Note: reduction is a chemical term for the addition of electrons; oxidation is the chemical term for the subtraction of electrons.

7. The big payoff of breaking down pyruvate for energy comes from oxidative respiration, where the pyruvate is broken down by the citric acid cycle (Krebs cycle). During this, energy is stored in the forms of ATP, and in the reduction of enzyme carriers NAD+ and FAD+ to NADH and FADH2. The electrons are then carried to the electron transport chain to the ultimate electron acceptor of oxygen. In the process, NADH and FADH2 are oxidized back to NAD+ and FAD+.

8. The Krebs cycle takes place in the matrix of the mitochondrion (the matrix is the material internal to the inner membrane). The pyruvate produced by cytoplasmic glycolysis easily diffuses towards the matrix. The pyruvate is hooked to Coenzyme A (CoA) to form acetyl-CoA. The acetyl CoA then enters the citric acid cycle, and in several"easy" steps two ATPs have been produced, as have six NADH and two FADH2, from each original glucose molecule. The carbons of the original pyruvates have all been oxidized and now are waste carbon dioxide.

9. All of these generated electrons (on the appropriate electron carrier) are then shunted off to the electron transport system on the inner mitochondrial membrane. On the inner mitochondrial membrane, the electrons give off energy and ultimately reduce (add electrons) to oxygen to create water. The hydrogen ions (protons) along the way are stored on the outer compartment side of the inner mitochrondrial membrane. As mentioned in # 4B, these hydrogen ions are pumped through an ATPase in that inner mitochondrial membrane to generate ATP. When all is taken into account (glycolysis, citric acid cycle, electron transport chain, chemiosmosis), the cell has generated (a net) 36 ATP for each glucose.

10. Although glucose, a sugar, was used in the example above, one can also burn fats and protein for energy production. Generally speaking, one has to break down the fat or the amino acid into a chemical appropriate for entrance into the glycolysis/citric acid pathway. Fats will supply about 9 kcal/gram, while proteins and carbohydrates will supply about 4 kcal/gram. Therefore, many animals will store energy in the form of adipose tissue.

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Please send comments and questions to: cronewil@hvcc.edu


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This page updated on November 22, 1999