Enzymes

[L.A. Moran]

In article <329E0503.3D57 at ibex.ca>,
Achim Recktenwald, PhD <achim at ibex.ca> wrote:
>David Moss wrote:
>> 
>> Melissa M <mirage at sympatico.ca> schrieb im Beitrag
>> <329D0764.7E23 at sympatico.ca>...

>> > I do not understand co-factors and co-enzymes. I understand the substrate
>> > and the active site. I also do not understand how ATP and NAD or NADH
>> > play a role in this. If you could explain this to me in simple terms it
>> > would be appreciated
>> 
>> You're probably getting confused by some irrelevant, historical
>> terminology.
>> A coenzyme (e.g. ATP, NAD) is simply a second substrate.
>[snip]
>
>For this reason they are often called 'co-substrates'. Actually, I
>prefer this expression to co-enzyme, since an enzyme is for me a
>protein.

Here's the terminology that we use:

     COFACTORS: chemicals required by inactive apoenzymes to convert 
                them into active enzymes. There are two types of 
                cofactors, essential ions and coenzymes.

        ESSENTIAL IONS: ions that are required for activity. Activator
                        ions are loosely bound while the metal ions of
                        metalloenzymes are tightly bound.

        COENZYMES: organic compounds required for activity. There are two
                        types of coenzyme; cosubstrates and prosthetic
                        groups.

             COSUBSTRATES: weakly bound to the apoenzyme often at a specific
                           site. Cosubstrates differ from other substates
                           in that they can be regenerated. Examples are
                           ATP, SAM, many vitamins, coenzyme A, THF, 
                           NADH, and ubiquinone (coenzyme Q).

             PROSTHETIC GROUPS: these are tightly bound to the apoenzyme
                           and if they participate in the reaction they
                           must be regenerated at every reaction cycle.
                           Examples are FAD, FMN, pyridoxal phosphate,
                           vitamin K and other vitamins (ie. vitamin A),
                           biotin, vitamin B12 (cobalamin) derivatives,
                           heme, and several modified amino acids. 

Hope this helps clarify the terminology for you. Other definitions are
posible but this seems to reflect the consensus in the field.

Melissa, please let us know if you need more information.



Larry Moran

[see Moran, Scringeour et al. BIOCHEMISTRY 2nd ed. Neil Patterson Publishers/
 Prentice Hall, Englewood Cliffs N.J. 1994; Chapter 8 - Coenzymes]