Enzymes #2

Michael Dorsett Onken mdonken at artsci.wustl.edu
Mon Dec 2 16:12:05 EST 1996


Melissa M (mirage at sympatico.ca) wrote:
: Hello,
:  
: I am still a little unclear about a few things. Okay is this right? An 
: enzyme cannot function unless there is a co-factor along with it (why is 
: that?).

This is partially true.  Not all enzymes use co-factors, but those that 
do, require the co-factors for their activity.

: There are two different co-factors metal ions and co-enzyme.

Co-factors can have two basic functions related to the activity of the 
enzyme and the type of co-factor:  

Metal ions can play two different roles:  they can help the enzyme hold 
its shape (like the Zinc in some DNA-binding proteins), or they can form 
part of the active site (like the Iron in hemoglobin, which directly 
binds to the oxygen being transported in the blood).  In either case, the 
metal ion is necessary for the enzymes activity, so it is considered a 
co-factor.

Co-enzymes are almost always involved in the active site of the enzyme, 
but they act more like substrates than enzymes.  Glycolysis uses a lot of 
co-enzymes, so I'll use two examples from it:
When converting  glucose to glucose 6-phosphate, ATP is converted to 
ADP.  What actually happens is the enzyme, glucokinase, binds to both ATP 
and glucose, and it holds them close together so that it can transfer a 
phosphate directly from the ATP molecule (making it ADP) to the glucose 
molecule (making it glucose 6-phosphate). So ATP is as much a substrate 
as a co-factor.
When converting glyceraldehyde 3-phosphate (G-3-P) into 1,3-diphospho- 
glycerate, (among other things) NAD+ is converted to NADH.  What actually 
happens is the enzyme, glyceraldehyde 3-phosphate dehydrogenase, binds to 
both NAD and G-3-P, and it holds them close together so that it can 
transfer a hydride ion (H-) directly from the G-3-P molecule (making it 
3-phosphoglycerate) to the NAD+ molecule (making it NADH).  So NAD+ is as 
much a substrate as a co-factor.

: There is something that causes a reduction to occur. What is that 
: process called??

Whenever electrons are transfered from one molecule to another, the 
molecule which loses electrons is "oxidized" (meaning it lost electrons), 
and the molecule which gains electrons is "reduced" (meaning it gained 
electrons).  Since a hydride ion has an extra electron, its transfer from 
G-3-P to NAD+ results in the G-3-P being oxidized (it lost an electron), 
and the NAD+ being reduced (it gained an electron).  This is a Redox 
Reaction (short for REDuction/OXidation).

: And also how does ATP and NAD and NADH evolve? I do not understand 
: this. It seems from my reading that they suddenly evolve, no 
: chemical reactions what so ever?

Most co-enzymes are taken in as vitamins (which is why vitamins are so 
important to a healthy diet) and converted into their active forms in the 
cell.  All co-enzymes and vitamins are synthesized through long 
biochemical pathways in some cells somewhere (usually in plants and 
bacteria).

: If you could help me it would be ever so appreciated for I have a test on 
: this on Tuesday!!!
:
: Melissa

I hope this helps,

Mike



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