Fatty Acid and Triglyceride Metabolism
|Depending on the energetic requirement of the organism, ingested
fats can be oxidized and used as fuel e.g. in muscle cells (myocytes) or stored in form of triglycerides (TG) in
fat cells (adipocytes). In both cases, the metabolism of dietary lipids is characterized by tightly defined stages
of TG hydrolysis and re-estrification processes.|
Dietary lipids are emulsified in the small intestine with the help of bile salts secreted by the gallbladder before the
TGs are hydrolyzed by lipases. The resulting degradation products can diffuse into the intestinal epithel cells where
they are reestrified to triacylglycerols (TG).
These TG are then packaged into lipoprotein transport particles called chylomicrons, which makes the TG suitable for
transport in the blood and lymph system. Lipoprotein lipase (LPL) hydrolyzes TG in chylomicrons and the released FA are
taken up into adipocytes where they are re-estrified to TG for storage.
In order to mobilize the stored energy, each TG is degraded step-by-step to three molecules of FA and glycerol. In the
first step, TG are hydrolyzed into diacylgycerols (DG) and FA. This reaction is catalyzed mainly by the recently
discovered enzyme 'Adipose triglyceride lipase' (ATGL) (Zimmermann et al, Science, 2004) and to a some extent by the
enzyme 'Hormone sensitive lipase' (HSL). The enzymatic activity of ATGL is drastically enhanced by CGI-58
(Lass et al, Cell Metab., 2006). The activity of HSL is controlled via the cyclic AMP dependent pathway involving
phosphorylation of HSL and perilipins, the 'gate-keepers' of the lipid droplet. HSL is also responsible for the
hydrolysis of DG to monoacylglycerol (MG) and FA. In the final step, monoacylgycerol lipase (MGL) hydrolyzes MG to
glycerol and FA. The produced molecules of FA are then released from the adipose tissue. The produced FA circulate in
the blood bound to a protein carrier, serum albumin, and are transported to energy requiring tissues. There, the FA are
activated and oxidized in response to the energy demand of the cell.
The challenging task of of hydrolyzing estrified lipids at the oil-water interface is carried out by enzymes called
lipases. Due to the different cellular localizations, cofactor requirements and other regulatory mechanisms, lipases
show a variety of sizes, substrate and positional specificities as well as different catalytic rates. Although the first
lipase 3D structures (human pancreatic lipase (HPL; Winkler et al., Nature, 1990) and Rhizomucor miehei lipase
(Brady et al., Nature, 1990) were elucidated already in 1990, detailed structures of numerous lipases,
questions regarding substrate specificities and interfacial activation remained unanswered.
Different structures of lipases revealed that most lipases belong to a superfamily of enzymes including esterases and
thioesterases. These lipases share an α/β-hydrolase fold with a central, mostly parallel, β-sheet. The mechanism
of hydrolysis is based on the catalytic triad as observed in serine hydrolases.
Structure of Horse Pancreatic Lipase
|Horse Pancreatic Lipase (Bourne et al, JMB, 1994) exhibits the typcial
α/β-hydrolase fold in its N-terminal catalytic domain. The catalytic triad, composed of residues Ser152,
Asp 176, and His263, is highlighted in magenta. The lid region is colored orange. The C-terminal domain is
responsible for binding of colipase and displays a β-sheet sandwich topology. |
Other lipases (e.g. ATGL) are members of a protein family harboring a patatin domain. The patatin domain is named after
the potato storage protein patatin, which posesses lipid acyl hydrolase activity. The catalytic mechanism of these
lipases proceeds via a catalytic serine-aspartate dyad consisting of the active site serine and an aspartate.
Structure of Patatin
|The 3-dimensional structure of the potato storage tuber protein patatin (Rydel et al, Biochemistry, 2003)
revealed a surface accessible Ser-Asp catalytic dyad (Ser77, Asp 215, magenta) |
The Catalytic Center and the Lid
|The catalytic triad of lipases with an α/β-hydrolase fold is composed of three amino acids
(Serine, Histidine, Aspartate/Glutamate), which are far apart in the primary sequence but spatially very close in the
folded protein. The interaction of the negatively charged residue Asp or Glu allows the His residue to act as a general
base which can remove a proton from the hydroxyl group of the active site Ser. The thus generated nucleophilic alkoxide
ion on the Ser residue is proposed to attack the carbonyl group of the estrified substrate forming an acyl-enzyme
intermediate. Another important component for the catalytic mechanism is the oxyanion-hole which is composed of properly
arranged H-bond donors (mostly main-chain NH groups). The oxyanion hole helps to stabilize a reaction intermediate
during catalysis when the carbonyl oxygen carries a partial negative charge.
The active serine residue of lipases is embedded in the short consensus sequence GXSXG (with X being any amino acid),
a motif also found in esterases, thioesterases and proteases. The active site of lipases in the 'closed' form is shielded
from the surface by protective surface loops called the 'lid'. Upon activation, the lid undergoes a conformational
rearrangement exposing the active site serine and creating the active, open form of the enzyme. Both, the open and the
closed form of lipases have been observed in X-ray structures of lipases.