Chemistry Reference
In-Depth Information
GPIs, also representing members of the glycolipid family, are covered in Chapter
9 of this topic, as noted above. Here, we now start the phylogenic overview with
glycolipids of photosynthetic bacteria.
10.2
Glycoglycerolipids in Thylakoid Membranes
The galactolipids monogalactosyldiacylglycerol ( MGDG ), digalactosyldiacylglyc-
erol (DGDG) and sulfoquinovosyldiacylglycerol (MGDG with a sulfonic acid group
on the 6-position of the galactosyl moiety) are found in all organisms performing
oxygenic photosynthesis. Functional analyses revealed that they are indispensable
for maximal effi ciency of photosynthesis. Owing to its high concentration in the
thylakoid membranes, MGDG is the most abundant glycolipid in nature. In chlo-
roplasts of plants MGDG and DGDG constitute about 50 and 30% of total lipids
in thylakoids, respectively [2]. A very similar glycolipid composition is found in
cyanobacteria. Its presence is in accord with the endosymbiont hypothesis, that is
by a photosynthetic ancestral cyanobacterium having been engulfed by a plant cell
as the origin of chloroplast formation. Of note, the function of thylakoid glycolipids
warrants further clarifi cation. Proper photosynthetic function is dependent on the
bilayer structure of the thylakoid membrane. Thus, a fi ne - tuned ratio between
MGDG and DGDG content of the thylakoid membrane is likely to establish the
appropriate biophysical characteristics of the membrane for photosynthetic func-
tion. However, biochemical studies suggested that thylakoid glycolipids specifi -
cally interact with protein complexes of photosynthesis, thereby promoting correct
assembly, long-range order and functional interplay of photosynthetic proteins.
Additionally, high-resolution crystallographic information obtained for photosyn-
thetic proteins has highlighted an intimate relationship between the multicompo-
nent protein-cofactor complexes of photosynthesis and thylakoid glycolipids.
These lipids resolved in X-ray crystal structures play roles in structural support,
bind cofactors, fi ll intraprotein cavities through which quinone (an important
electron acceptor in electron transport chains of photosystems I and II of photo-
synthesis) can diffuse, are important for the stabilization of multimeric complexes
and may lubricate complexes that need to be structurally fl exible. It is possible that
particular glycolipids can infl uence the biophysical properties of the cofactors,
especially in the case of quinones, and so affect parameters such as the rate of
electron transfer [5]. Glycoglyceroglycolipids are also found in bacteria without
capacity for photosynthesis.
10.3
Glycolipids in Non- photosynthetic Bacteria
Glycoglycerolipids are widespread in Gram-positive bacteria. The degree of their
structural variability surpasses that in photosynthetic bacteria by far. The principal
