concepts in biochemistry
WILLIAMM. SCOVELL edited by Bowling Green State University Bowling Green. OH 43403
Biological Membranes Function and Assembly Sepp D. Kohlwein lnstitut fur Biochemie und Lebensmitlelchemie. Technische Universitat Graz, Petersgasse 12A1,A 8010 Graz, Austria Biological membranes are essential boundaries within the living cell. Plasma membranes separate the interior of the cell from the environment and provide means for intercellular communication. Intracellular membranes divide the cell into distinct compartments that have separate catabolic a n d anabolic functions: t h e nucleus, mitochondrion, peroxisome, endoplasmic reticulum, and vacuoles. Membrane research has suffered from experimental problems inherent in the analysis of membrane components. For example, i t is extremely difficult to retain the active state of membrane proteins when isolating them, thus lowering the likelihood that they will be analyzed. This limit in our access to active and isolated membrane components stands in strong contrast with the vast number of water-insoluble membrane lipids (phospholipids) that form the membrane matrix. The biochemical and genetic analysis of unicellular prokaryotic and eukaryotic microorganisms bas contributed significantly to our understanding of membrane lipid synthesis, intracellular lipid movement, and the incorporation of proteins into membranes. Examples include recent work done with the prokaryote Escherichia coli ( 1 ) and with the eukaryote yeast Saccharomyces cerevisiae (2). Since these microorganisms can be readily manipulated biochemically and genetically, they allow us to unravel some very complex cellular processes: the pathways involvedin the synthesis of membrane lipids and proteins; and the regulation of membrane assembly. New techniques in molecular genetics have provided the experimental tools needed for cloning and characterizing the genes t h a t encode membrane proteins. Using the primary structures derived from DNA sequencing, experts have had some success predicting the conformational structures and domains of the gene products. On the other hand, the isolation of individual phospholipid classes or the chemical synthesis of these lipids and their reconstitution in uitro h a s allowed detailed analysis of the molecular properties and dynamics of phospholipid molecules in artificial membranes. Now that the experimental tools a r e available, phospholipid a n d membrane properties can be studied in increasingly complex reconstituted systems, in isolated cellular fractions, or in whole cells. I n his 1980 article "Fashions in membranes", Aser Rothstein (3) extrapolated that by the year 2005 all fulltime members of the medical schools would be involved in membrane research. He based this on the publications on membrane research that had been published since 1948. Reviewing the literature today gives the impression that we might have reached this state already.
I n this review we intend to summarize some of the methodology involved i n biological membrane research and some of the recent developments in our understanding of how biological membranes are assembled and how they function. Introduction to the Membrane Concept Until 1972 biological membranes were generally considered to be static structures. Mainly through the use of spectroscopic techniques this picture has changed to the present, well-established "fluid mosaic model" (4). In a n aqueous environment, phospholipid molecules (Fig. l a ) spontaneously aggregate and form bilayers (Fig. Ib) due to their amphipathic character. This bilayer structure minimizes the contact between the hydrophobic side chains of the phospholipids and the surrounding water molecules ('Onydrophobic effect"). The thermodynamic stability of the membrane structure is also due to other interactions: hydrophobic interactions among the fatty acyl chains eleetmstatic interactions between zwitterionic head groups and water molecules and other ions eleetmstatic interactions between negatively charged head groups and water molecules and other ions The model implies that phospholipid molecules a s well a s other components of a biological membrane, such a s proteins or-sterols, are subjectto various types of movements: rotational movement limited oscillation in and out of the plane of the membrane lateral diffusionwithin the membrane layer Transverse movement of the polar phospholipids from one leaflet of the bilayer to the other, called a flip-flop, is extremely unfavored thermodynamically in pure phospholipid membranes: The half-lives for transverse movements a;e usually several days. However, a class of enzymes called flippases have recently been discovered in biological membranes. They are involved either in the randomization of phospholipid distribution or in the maintenance of asymmetry in phospholipid distribution between the two leaflets of the bilayer. (For a review see ref 5.) The "fluid mosaic model" has been proven to be valid for many different cellular systems and subcellular compartments. However. nonbilaver structures have also been identified. Hexa&nal (HI,") phases, shown in Figure lc, have been identified a s the tvDe ". of membrane structure assumed when certain lipids are incorporated, for example, phosphatidylethanolamiue or cardiolipin. I t has also been postulated that their formation can be triggered by the presence of certain ions (for example, Ca2+). Volume 69 Number 1 January 1992
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The formation of lipid and protein clusters that limit the lateral diffusion of individual membrane components is a n important feature of biomembranes. These clusters can be stabilized by interactions between lipids and proteins. They play an essential role in the organization and regulation of complex biochemical pathways, such a s the respiratory chain responsible for energy conversion. Bioscientists are now i n a position to determine the int e r r e l a t i o n between membrane proteins and membrane lipids a t the molecular level. This will lead to a n understanding of how lipids contribute to the functioning of membrane proteins.
head p u p fatty a q l chains (polar) lapolar, hydrophobic) R
R1,R2 :fatty 8eyl chains, predominsnteiyC 1 ~ - C z 0 . saturated or unsaturated
F gure 1 a (rlght]: General strLctura formJ a of glycerophospnop os Alcoho grodps in post ons s n l ano sn-2 of the g ycero backbone are acylated or alkylated with hydrocarbon chains that are saturated or unsaturated and preferentiallycontain 1 6 2 0 carbon atoms. Aphosphate group, which is negatively charged at neutral pH, is esterified with the hydroxyl group at position sn-3 (phosphatidic acid). Various charged or polar head groups (X) are linked via ester bonds to the phosphate group. Charged head groups give a zwitterionic compound as seen in phosphatidylethanolamine and phosphatidylcholine. Polar head groups give an acidic compound. a s seen in ~hosphatidvlinositol, phosphalrdylser ne, phospnaI oy g ycerol, ano dlpnospnal!aylglycero ,cardo p n~pnospno pds
Introduction to Membrane Function Membrane Proteins
The hydrophobic barrier of t h e bilayer would cause a purely phospholipid membrane to be essentially impermeable to charged and polar molecules. However, t h e protein-catalyzed membrane transport of solutes or metabolites enables t h e cell to adapt to alterations in the environment. Energy-driven ion pumps and pore proteins establish the membrane potential and maintain a strictly controlled ionic milieu within t h e cell a n d w i t h i n membrane-enclosed subcellular organelles. They also allow rapid changes in the internal ion concentration in response to external stimuli. for exduring trans- b (left):Arrangement of phospholipids and proteins in biological membranes. In aqueous media, phosduction in the sys- pholipids form bilayer structures, which comprise the membrane matrix. Proteins may be attached tem. peripherally or they may extend into or transverse the membrane bilayer. The translation of extracel- (right):Nonbilayer structure of phospholipids-hexagonal (HI,)phase. lular signals into coordinated growth and differentiation rewhere the information they contain is translated into quires the presence of many proteins with different funcprotein. Thus, spatial separation of DNAtranscription and tions: receptor proteins bound to the plasma membrane, RNA translation in eukaryotic cells provides a means of membrane-associated protein kinases, or GTP-binding cellular regulation. It also allows the generation of difproteins. Mutations in the genes, which encode these facferent proteins from a single structural gene. torsand the result of which is called a n oncok-ene, may lead to uncontrolled growth and cancer development in 6igher The highly specialized functions of chloroplasts and eukaryotes (e.g., erbB, src, ras). mitochondria are performed by specific proteins that are embedded in a matrix of membrane lipids. Chloroplasts Membrane-Enclosed Organelles are involved in COz fixation and energy production in green algae and plants. Mitochondria are responsible for The spatial separation of anabolic and catabolic procesoxidative energy production. The membrane itself is an esses in membrane-enclosed organelles of eukaryotic cells orsential component of these processes because it allows the ~ a n i z e scomulex cellular Drocesses. For examole, the generation of a proton gradient or membrane potential enclosure ofthe genetic matcnal in the nucleus by a double that can be translated via ATPases into metabolic energy rnembrime ocrmlts extenswe modification of primary RNA in the form of ATP (6). transcripts'before they are transferred to the cytoplasm,
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Journal of Chemical Education
Membrane Lipids
However, evidence continues to emerge that membrane lipids do more than form a structural matrix: Some lipid ciasses selectively modulate t h e activity of specific membrane proteins. It h a s become apparent t h a t a mitochondria1 enzyme, cytochrome c oxidase, may be dependent on cardiolipin. This is based primarily on in uitm d a t a , b u t also on in uiuo analyses (7).Lill e t al. (8) demonstrated that in E. coli acidic phospholipids stimulate the translocation of the SecA gene product and its ATPase activity into the inner membrane. Several classes of ohos~holioidsare involved in vital orocesses, such a s signal transduction. For example, pho~phatidyl~erin was found to stimulate protein kinase C in a Ca2+-dependent manner (9). Turnover of hieher ohosohorvlated ohos~hoinositides . . leads to the intract!llulnr release of the sramd messengers, inositol 1.4.5-trisohosuhatc and diacvl~lscerol. They turn .. . -. on a whole subset of regulatory processes b the activation of orotein kinases and the release of Ca from the endoplasmic reticulum ( 1 0 1 . I
