fatty acids and glycerol, protein amino acids, etc.)
| Chem 431 |
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Fall 2007 |
| Lecture Notes: 24 October |
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| PREVIOUS |
Introduction to Metabolism
Catabolism: degradation of molecules to provide energy.
Anabolism: reactions using energy to synthesize new molecules for growth etc.
(overhead - Interactions of Metabolic Pathways; text Figure PtII-3,4) sequences of consecutive enzyme catalysed reactions which are readily studied and traced. A more rational definition is that of Newsholme and Leach (Biochemistry for the Medical Sciences, Wiley, 1983: pg.42)
"[A] metabolic pathway is a series of enzyme-catalyzed reactions, initiated by a flux-generating step and ending with either the loss of products to the environment, to a stored product (a metabolic 'sink') or in a reaction that precedes another flux-generating step (that is, the beginning of the next pathway)." Where a flux generating step is a non-equilibrium reaction that generates the flux going through the pathway and to whose rate all other reactions of the pathway conform. Note that by this definition some pathways may be inter-organ while others may take place in single compartment. We will explore this definition/concept as we look at metabolism.
Characteristics of pathways:
The flux through a metabolic pathway is invariably controlled or regulated, most commonly by Feedback Inhibition, but also through Feed-forward activation. Regulation is one of the things that makes biochemistry "biological" and it will be a focus in our study.
The Stages of Catabolism [overhead]: For convenience we can breakdown catabolism into four hierarchical levels:
fatty acids and glycerol, protein amino acids, etc.)
Organic Reaction Mechanisms: We can categorize all common biological reactions into four groups:
Look at ATP. In the figure the bolded region is the "recognition" part of the molecule, while the polyphosphate is the chemically active portion. Each of the phosphoric acid anhydride bonds is unstable. That is hydrolyzing either will release a lot of energy. (text Figure 1-25)
So why ATP? First, we want a compound with intermediate hydrolysis energy so it can pick up energy from some reactions and deliver to others. Second we want a kinetically stable molecule which is thermodynamically unstable. Thus acetic acid anhydride would not work: it is thermodynamically unstable to hydrolysis, but it is also kinetically unstable, with the carbonyl carbons wide open to water attack. Phosphoric acid anhydride is equally unstable, but is is sterically protected from water attack - in order to react quickly we need a catalyst - perfect.
ATP is sometimes referred to as a "Hi Energy" compound. High energy in this case does not refer to total energy in compound, rather just to energy of hydrolysis. Thus ATP is unstable to hydrolysis, or has a large negative deltaG for hydrolysis. For biochemistry High Energy is defined in terms of ATP: if a compound's free energy for hydrolysis is equal to or greater than ATP's then it is "High Energy," if its free energy of hydrolysis is less than ATP's then it is not a "hi energy" compound. Note that ATP has two hi energy anhydride bonds.
You should memorize the structures for ATP, ADP, & AMP.
 Pathway Diagrams |
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Last modified 24 October 2007
