Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online

Structure & Reactivity in Chemistry

Metabolic Pathways

MP1. An Overview of Metabolic Pathways - Catabolism

 Henry Jakubowski

Biological cells have a daunting task.  They must carry out 1000s of different chemical reactions required to carry out cell function.    These reactions can include opposing goals such as energy production and energy storage, macromolecule degradation and synthesis, and breakdown and synthesis of small molecules.  All of these reactions are catalyzed by proteins and RNAs enzymes whose activities must be regulated, again through chemical reactions, to avoid a futile and energy wasting scenario of having opposing pathways functioning simultaneously in a cell.


Metabolism can be divided into two main parts, catabolism, the degradation of molecules, usually to produce energy or small molecules useful for cell function, and anabolism, the synthesis of larger biomolecules from small precursors.  


CATBOLISM:   Catabolic reactions involve the breakdown of carbohydrates, lipids, proteins, and nucleic acids to produce smaller molecules and biological energy in the form of heat or small thermodynamically reactive molecules like ATP whose further degradation can drive endergonic process such as biosynthesis.  Our whole world is reliant on the oxidation of organic hydrocarbons to water and carbon dioxide to produce energy (at the expense of releasing a potent greenhouse gas, CO2).  In the biological world, reduced molecules like fatty acids and partially oxidized molecules such as glucose polymers (glycogen, starch), as well as simple sugars, can be partially or fully oxidized to ultimately produce CO2 as well.  Energy released from oxidative reactions is used to produce molecules like ATP as well as heat.   Oxidative pathways include glycolysis, the tricarboxylic acid cycle (aka Kreb's cycle) and mitochondrial oxidative phosphorylation/electron transport.  To fully oxidize carbon in glucose and fatty acids to carbon dioxide requires splitting C-C bonds and the availability of series of oxidizing agents that can perform controlled, step-wise oxidation reactions, analogous to the sequential oxidation of methane, CH4 to methanol (CH3OH), formaldehyde (CH2O) and carbon dixoxide.

Figure:  Summary of Glycolysis

Brief Summary Glycolysis

Figure:  Pyruvate Dehydrogenase (mitochondrial) and the TCA Cycle

text insert TCA

Figure:  Mitochondrial Electron Transport/Oxidative Phosphorylation

textinsert OxPhos Mito

Feeder Pathways:  Other catabolic pathways produce products that can enter glycolysis or the TCA cycle.  Two examples are given below.

textinsert glycogenolysis

textinsert fa oxidation

textinsert prot degrad

Now on to anabolic reactions.


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Biochemistry Online by Henry Jakubowski is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.