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Samford University -- Department of Biological and Environmental Sciences
Cell and Molecular Biology -- Biol 405

<<<  Metabolism  >>>

The myriad of chemical reactions occurring in the cell are collectively called metabolism. Metabolism can be divided into two categories: 1) catabolism, the breakdown of molecules, and 2) anabolism, the synthesis of new molecules. We will look at two aspects of metabolism: 1) enzymes, and 2) energy and metabolism. (We will cover some aspects of the last section of chapter 3, biosynthesis, later.)

  • Enzymes: Enzymes are protein catalysts. (Note: there are also catalytic RNAs.) A catalyst increases the rate of a chemical reaction without itself being permanently changed. Enzymes bind (lock and key binding) to a substrate (or substrates). After the reaction, an end product (or end products) is (are) released. While the enzyme may have been altered in the intermediate substrate-bound stage, it is released unaltered and can be reused. Therefore, a small amount of enzyme can catalyze numerous reactions.

Enzymes are things invented by biologists that explain things which otherwise require harder thinking.        -- Jerome Lettvin








    • Coenzymes: A coenzyme is a type of prosthetic group (small, non-protein molecules bound to proteins, like heme). Coenzymes are small organic molecules that further increase reaction rates. One example is the nucleotide called nicotinamide adenine dinucleotide (NAD+) which can be reduced (to NADH) by receiving one H+ and two electrons and then oxidized (back to NAD+) by donating these to another molecule. The reduction/oxidation of NAD+/NADH is coupled to the oxidation/reduction of other molecules. NAD+ ---> NADH requires energy. NADH ---> NAD+ releases energy. (Video) Similarly, the nucleotide called flavine adenine dinucleotide (FAD) can be reduced to FADH2 (requiring energy) and FADH2 can be oxidized to FAD (releasing energy).
    • Enzyme Inhibition/Activation: Enzymes are often inactivated by the presence of a pathway end product that binds to a site other than the catalytic site (allosteric binding). This is feedback inhibition. (F.E. Video.)  Enzymes may also be activated or deactivated by the addition of phosphate groups to a serine, theonine, or tyrosine amino acid side chain. This is phosphorylation. The activity of enzymes responsible for phosphorylation (a protein kinase) may in turn be regulated by phosphorylation (another protein kinase). (Video at 5:50) Enzymes that remove phosphates are called protein phosphatases.


  • Energy and Metabolism: In cells, energy-rich molecules, like carbohydrates and lipids, are catabolized releasing energy which is captured in a usable form in the nucleotide called adenosine 5'-triphosphate (ATP). Various catabolic reactions are coupled with the synthesis of ATP from ADP + PO4--. ATP is the energy currency of the cell and can be used for various cellular activities. Energy is released when ATP is hydrolyzed to ADP + PO4--. Other catabolic reaction are coupled to the production of NADH from NAD+ or of FADH2 from FAD. Also, guanosine 5'-triphosphate (GTP) may substitute for ATP. (Energy in the form of GTP can be considered equivalent to energy in ATP: GTP can react with ADP to yield GDP and ATP).
    • Glycolysis: This initial breakdown of a glucose molecule is anaerobic and occurs in the cytosol. It converts the 6 carbon glucose molecule into two 3 carbon molecules (pyruvate). It requires the hydrolysis of two ATPs during the initial steps (activation), but yields two ATPs per pyruvate, so there is a net gain of 2 ATPs directly from glycolysis. Also, during glycolysis one NADH is made per pyruvate, yielding 2 NADHs. (Video)
    • Acetyl CoA Production and the Citric Acid Cycle (Kreb's Cycle, Tricarboxylic Cycle): Pyruvate crosses the mitochondrial membrane and the rest of the breakdown occurs within the mitochondrion. This process is aerobic.
      • Acetyl CoA: As the 3 carbon pyruvate is entering the mitochondrion, it is converted to a 2 carbon acetate with the addition of coenzyme A (CoA) and the release of a carbon in the form of a CO2 molecule. In the process, energy again is captured with the reduction of one NAD+ to NADH per pyruvate. (Pyruvate + CoA + NAD+ ---> Acetyl-CoA + CO+ NADH)
      • Citric Acid Cycle: In the matrix of the mitochondrion, the complete break down the acetate to CO2 occurs through a series of reaction. At various steps, energy is captured with the net gain of 1 ATP, 3 NADHs, and 1 FADH2 per acetyl CoA. (Video)


    • Electron Transport Chain: The energy captured in NADHs or FADH2 is converted into ATP energy trough the electron transport system which occurs on the inner mitochondrial membrane. This is oxidative phosphorylation and involves the oxidation of NADH and FADH2. In this chain of reactions, oxygen serves at the final electron acceptor and each NADH yields 3 ATPs while each FADH2 yields two. (Video)
  • The breakdown of a glucose is just one example of an energy-producing catabolic set or reaction. All of these reactions (anabolic and catabolic) are interconnected* in cell metabolism.
*University of Strasbourg

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