Clever enzyme regulates ammonia

[Fred56]

An enzyme (NAGK) used by a cyanobacterium allows a configuration of arginine units (ammonia storage) that means they don't bind too tightly, so the arginine units remain available:

 

BIOCHEMISTRY: Three Times Two Is Six

Gilbert J. Chin

 

Photosynthetic organisms take advantage of an abundance of nitrogen by storing it as arginine (whose side chain contains a nitrogen-rich guanidinium group), a reaction that is catalyzed by the dimeric protein N-acetylglutamate kinase (NAGK)--an enzyme that is, not surprisingly, subject to feedback inhibition by arginine. PII proteins are one of the central metabolic coordinators of carbon and nitrogen fluxes; this protein is a homotrimer and has been shown previously to regulate ammonia influx into cells by inserting its T loop into the cytoplasmic vestibule of the trimeric ammonia channel; too much of a good thing can be hazardous to one's acid/base equilibrium.

 

Llácer et al. describe structural and biochemical studies of the interaction between PII and NAGK in the cyanobacterium Synechococcus. They show that a hexameric ring (a trimer of dimers) of NAGK is sandwiched by a pair of PII trimers, so that each of the six PII subunits makes a reversible contact covering about 600 Å2 with a single NAGK subunit. Each NAGK dimer is oriented at an angle to the plane of the ring, as in the blades of a propeller, and the binding of PII increases the angle slightly. The arginine-binding sites are located at the interdimer surfaces, and tilting the dimers and altering the interdimer contacts reduces the binding affinity for arginine by 15-fold. Unlike its direct inhibition of the ammonia channel, the effect of PII binding on NAGK, though also mediated in large part by the T loop, is entirely indirect and distant from both the catalytic and allosteric sites.-- GJC

 

Proc. Natl. Acad. Sci. U.S.A. 104, 17644 (2007).

[Phi for All]

Please, as the opening poster, provide some discussion points. Calling our attention to articles we can find ourselves is unnecessary. Please set the tone and the stage for a productive discussion.

[Fred56]

This is something I posted here because there didn't seem to be anywhere else. Is the rule meant to be that everything has to ask a question or set the stage for discussion?

Perhaps I could have asked if anyone thinks we are getting close to completing the map of biochemical pathways, or something? We seem to be improving the resolution -this was one of several articles about biochemistry from an email I got.

What I thought was "interesting" was the indication that there is an apparent adaptation to the "behaviour" of stereochemical compounds, and how the NAGK-PII complex "modifies" the binding affinity. This probably evolved because of the behaviour of so-called "self assembling" protein-units -so the most efficient would have emerged eventually. It's an example of co-option of conformal properties of such (stereochemical) things by evolution, maybe.

 

the article:

 

http://www.pnas.org/cgi/content/abstract/104/45/17644