The rates of reactions catalyzed by immoblized enzymes has been a subject of
concern for decades. As pointed out by Paul Schlosser, Douglas Kell, et al.
restricted diffusion to the site of the enzyme alters the local concentration
of substrate in an "unstirred layer." But there is another interesting
component; enzymes in a pathway may be membrane bound with adjacent proteins
catalyzing sequential steps. The most common example is in mitochondria, but it
was reported by an Australina lab in Biochemical Journal (I think) for muscle
contractile filaments about 20 years ago. The glycolytic enzymes plus malate
dehydrogenase line up along actin filaments. So NADH formed at glyceraldehyde
phosphate dehydrogenase, for example, is reoxidized by lactate dehydrogenase
and malate dehydrogenase localized nearby on the filament. Such "privileged
access" or "channeling" of substrates has been the focus of much research in
recent years.
I encounter similar cases in my own work involving metabolism of xenobiotics by
inducible enzymes bound to the hepatic endoplasmic reticulum. Olefinic
chemicals are oxidized by several isoforms of cytochrome P450 to epoxides,
which are converted to vicinal diols by the microsomal enzyme epoxide
hydrolase. The net rate of accumulation of the epoxide intermediate cannot be
reproduced by equations that treat these two enzymes as independent. Hydrolysis
of an endogenously produced epoxide is more rapid than observed when
"preformed" epoxide (not its precursor) is the exogenously supplied substrate.
Johannes Filser in Germany treats this endogenous epoxide intermediate as
localized in a hypothetical subcellular compartment. I follow Douglas Kell's
idea of an intermediate which diffuses laterally along the endoplasmic
reticulum membrane faster than it diffuses into the bulk cytoplasm and
therefore encounters the hydrolase faster than it would if exogenously
supplied.
--
Michael C. Kohn
Laboratory of Computational Biology and Risk Analysis
National Institute of Environmental Health Sciences
P.O. Box 12233, Mail Drop A3-06
Research Triangle Park, NC 27709-2233
Telephone:
919-541-4929 (voice)
919-541-1479 (fax)
e-mail:
kohn at valiant.niehs.nih.gov
WWW home page:
http://valiant.niehs.nih.gov
