Enzymes catalyze metabolic reactions. What is a catalyst? What is activation energy? What does activation energy have to do with a catalyst?

Enzymes bind to substrate molecules and force them into "uncomfortable" positions. These "uncomfortable" substrate molecules have slightly distorted electron orbital configurations and this make them reactive. Thus we say that the substrate molecules turn into reactive intermediates. The reaction then occurs and the product is released. The enzyme then binds more substrate molecules.

What is meant by competitive inhibition?

Some substrates have natural analogs which can bind the active site (the substrate binding site) of an enzyme. When this happens, the analog, not being the substrate, will not react. Since it won't react, it won't pop out of the active site either. This enzyme becomes stuck or poisoned by the analog. The most commonly cited example of this involves the reaction:

paraminobenzoic acid (paba) ------> folic acid

sulfanilamide, a synthetic antibiotic, is an analog of paba and will compete with paba for the binding site on the enzyme which catalyzes this reaction.

A group of enzymes known as allosteric enzymes have two binding sites: (1) a substrate binding site (the active site) and (2) a cofactor binding site (the allosteric site). In these enzymes the binding of the cofactor to the allosteric site will change the conformation of the enzyme. Depending on the enzyme, this change can either open or close the active site. Thus the activity of allosteric enzymes is controlled by the concentration of cofactors. Some common cofactors include: Ca++, Mg++, Fe++ or +++, cAMP, and PO4---.

In metabolic reactions energy flows in terms of electrons. Electrons are gained and electrons are lost. Oxidation is the loss of electrons. Reduction is the gain of electrons. Oxidation and reduction are coupled. In the cell a number of electron carrier molecules such as NAD, NADP and FAD serve to move electrons around between reactions.

In the following reaction, reading from left to right, lactic acid is oxidized to form pyruvic acid and NAD gets reduced in the process. (Of course if you read from right to left, pyruvic acid gets reduced to lactate and NADH2 serves as the electron donor).

The ultimate goal in many cases is to take the energy and make ATP.

There are three ways that ATP is made:

1. Substrate level phosphorylation - the ATP made in glycolysis is made this way.

2. Oxidative Phosphorylation - The ATP made during respiration is made this way.

3. Photophosphorylation - the ATP made during photsynthesis is made this way.

Summary of Glucose Metabolism:

1.) Glycolysis - Glucose (6 carbon sugar) is partially oxidized into 2 pyruvates (2, 3-carbon molecules). In the process, 2 ATP's are consumed and 4 ATP's are made. 2 NADH2's are also made.

Pyruvate can go into either Fermentation or Respiration:

             FERMENTATION                              RESPIRATION                
1. An organic molecule is the final      1. An inorganic molecule, usually O2,    
electron acceptor.                       is the final electron acceptor.          
2. Anaerobic process                     2. Aerobic if O2 is terminal electron    
                                         acceptor; if not it is anaerobic.        
3. There neither a krebs cycle nor       3. The krebs cycle operates as well as   
electron transport.                      an electron transport chain.             
4. The ATP is made during glycolysis     4. Most of the ATP is made by            
                                         chemiosmosis driven by the energy        
                                         released in the electron transport       
                                         chain.                                   
5. NAD is regenerated from NADH2 by      5. NAD is regenerated by the reduction   
the reduction of an organic molecule.    of the electron transport chain.         
6. The products of fermentation          6. The products of aerobic respiration   
include:  lactic acid, formic acid,      are: CO2 and water and alot of ATP.      
ethyl alcohol, isopropyl alcohol,                                                 
acetic acid, butyric acid, butanol,                                               
acetoin, acetylmethylcarbinol.                                                    
Different species produce different                                               
combinations of these compounds and so                                            
these products can be very important                                              
in helping to ID the organism.  Of                                                
course there is a relatively small                                                
amount of ATP made also.                                                          

Organisms are often classified according to their energy and carbon requirements. Thus:

Phototrophs - are photosynthetic

Chemotrophs - get energy through the oxidation of chemical compounds

Autotrophs - get their carbon from CO2

Heterotrophs - get their carbon from organic molecules