Mannose-6-phosphate (M6P) is a molecule bound by lectin in the

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...

. M6P is converted to fructose 6-phosphate by

mannose phosphate isomerase Mannose-6 phosphate isomerase (MPI), alternately phosphomannose isomerase (PMI) () is an enzyme which facilitates the interconversion of fructose 6-phosphate (F6P) and mannose-6-phosphate (M6P). Mannose-6-phosphate isomerase may also enable the s ...

. M6P is a key targeting signal for

acid hydrolase An acid hydrolase is an enzyme that works best at acidic pHs. It is commonly located in lysosomes, which are acidic on the inside. Acid hydrolases may be nucleases, proteases, glycosidases, lipases, phosphatases, sulfatases and phospholipas ...

precursor proteins Precursor or Precursors may refer to: *Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of unr ...

that are destined for transport to

lysosome A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane p ...

s. The M6P tag is added to such proteins in the ''cis''-

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles i ...

. Specifically, in a reaction involving

uridine diphosphate Uridine diphosphate, abbreviated UDP, is a nucleotide diphosphate. It is an ester of pyrophosphoric acid with the nucleoside uridine. UDP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase uracil. UDP is an im ...

(UDP) and ''N''-acetylglucosamine, the enzyme

N-acetylglucosamine-1-phosphate transferase N-acetylglucosamine-1-phosphate transferase is a transferase enzyme. Function It is made up of two alpha (α), two betas (β), and two gammas (γ) subunits. ''GNPTAB'' produces the alpha and beta subunits, ''GNPTG'' produces the gamma subunit. ...

catalyzes the ''N''-linked glycosylation of

asparagine Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...

residues with M6P. Once appropriately marked with the M6P targeting signal, these proteins are moved to the ''trans''-Golgi network. There, the M6P

moiety Moiety may refer to: Chemistry * Moiety (chemistry), a part or functional group of a molecule ** Moiety conservation, conservation of a subgroup in a chemical species Anthropology * Moiety (kinship), either of two groups into which a society is ...

is recognized and bound by mannose 6-phosphate receptor (MPR) proteins at pH 6.5–6.7. The M6P-tagged lysosomal enzymes are shipped to the late

endosomes Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membrane can ...

via vesicular transport. Enzyme replacement therapy (ERT) for several

lysosomal storage diseases Lysosomal storage diseases (LSDs; ) are a group of over 70 rare inherited metabolic disorders that result from defects in lysosomal function. Lysosomes are sacs of enzymes within cells that digest large molecules and pass the fragments on to other ...

relies on this pathway to efficiently direct synthetic enzymes to the lysosome where each can metabolize its particular substrate. The pH in the late endosome can reach 6.0, which causes dissociation of M6P from its receptor. Upon release, the enzymes are ferried to their final destination in the lysosomes. The MPRs are packed into vesicles that bud off the late endosome and return to the ''trans''-Golgi network. In this way, the MPRs can be recycled.

References

External links

*
Role of M6P in protein modification(video)
{{Fructose and galactose metabolic intermediates Monosaccharide derivatives Organophosphates

See also

References

External links