Lipids matrix, function membrane proteins

The structure of the photosynthetic membrane in com m on with all biological membranes is considered to be a lipid bilayer matrix to which the various functional membrane proteins are associated. The form of the association can be intrinsic to the lipid component in which case there is presumed to be contact between the polypeptide chains of the proteins and the hydrophobic domain of the lipid. Other proteins are believed to be peripheral to the lipid bilayer and associate mainly through electrostatic interactions. 

The calcium-independent ATPase of the lipid modified preparations is not only different from the calcium-dependent ATPase but also from the calcium-independent ATPase of native preparations — the basic ATPase — which has a lower nucleotide specificity126. The experiments in which the lipid matrix of the sarcoplasmic membranes has been replaced by lipid compounds not present in native membranes reveal a high degree of functional flexibility of the enzyme. On the other hand, a few residual lipids in the protein are sufficient to prevent these changes in the structure of the enzyme and to preserve its calcium sensitivity. 

Membranes of plant and animal cells are typically composed of 40-50 % lipids and 50-60% proteins. There are wide variations in the types of lipids and proteins as well as in their ratios. Arrangements of lipids and proteins in membranes are best considered in terms of the fluid-mosaic model, proposed by Singer and Nicolson % According to this model, the matrix of the membrane (a lipid bilayer composed of phospholipids and glycolipids) incorporates proteins, either on the surface or in the interior, and acts as permeability barrier (Fig. 2). Furthermore, other cellular functions such as recognition, fusion, endocytosis, intercellular interaction, transport, and osmosis are all membrane mediated processes. 

Biological membranes are always pictured as being very selective barriers separating different biochemical reaction compartments. This high performance transport specificity solely depends on the presence of membrane proteins embedded in the lipid matrix. On the other hand, most membrane proteins cease to function in the absence of lipids. In order to introduce biological transport abilities into artificial membrane systems protein-lipid interactions are of vital interest. The question is how the activity of membrane proteins is affected if they are placed into a polymeric environment. 

Studies have suggested that membrane proteins do not always undergo free diffusion in the membrane, and that they may be anchored by proteins of the cytoskelton, by the extracellular matrix or in other ways. Also, membranes are not uniform in their distribution of components, even on one side of the bilayer. They contain many specialized regions or domains, such as clathrin-coated pits, synapses in nerve cells, microvillae, and focal contacts. The lipid and protein composition of the domains are distinct and have been associated with specialized biological functions of the membrane. 

A steady flow of metabolites both in and out of the mitochondrial matrix space is necessary for mitochondria to perform functions which involve the participation of enzymes inside the membrane permeability barrier. These functions include oxidative phosphorylation and therefore O2, ADP, phosphate and electron-rich substrates such as pyruvate, fatty acids and ketone bodies must enter the mitochondria, and the products, HjO, CO2 and ATP must leave. Although Oj, HjO and CO2 are permeable to the inner mitochondrial membrane [1,2], most metabolites are not, because of their highly hydrophiUc nature. The outer mitochondrial membrane does not present a significant barrier to hydrophilic metabolites because of the presence of large unregulated channels composed of the membrane protein, porin [3]. The inner mitochondrial membrane has a much larger surface area [4] than the outer membrane and a much higher ratio of protein to lipid [5]. It is composed not only of proteins involved in electron transport and oxidative phosphorylation but also specialized proteins which facilitate, and in many cases provide, directionality to the transport of metabolites [6]. 

A large number of examples have been outlined above where these structures occur spontaneously, and their transformations between flat and curved forms are evidently used expensively in nature. The approach to thinking about structure and function in terms of membrane curvature presented here allows insights into biological function that are inaccessible through the conventional view of membranes, that largely focuses on the role of proteins, to the exclusion of the lipid matrix. 

Lipids are not covalently bound in membranes but rather interact dynamically to form transient arrangements with asymmetry both perpendicular and parallel to the plane of the lipid bilayer. The fluidity, supermolecular-phase propensity, lateral pressure and surface charge of the bilayer matrix is largely determined by the collective properties of the complex mixture of individual lipid species, some of which are shown in Fig. 8.1. Lipids also interact with and bind to proteins in stiochiometric amounts affecting protein structure and function. The broad range of lipid properties coupled with the dynamic organization of lipids in membranes multiplies their functional diversity in modulating the environment and therefore the function of membrane proteins. 

Lipids have multiple functions in cells ranging from defining the bilayer permeability barrier of cell membranes and organelles to providing the matrix within which membrane proteins fold and function to being integral components of... 

The current opinion, widely held, is that all biological membranes, including mammalian plasma membranes, have as a structural framework a phospholipid bilayer of which the characteristic feature is a parallel array of hydrocarbon chains, averaging 16 carbon atoms in length. This bilayer has some of the properties of a two-dimensional fluid in which individual lipid molecules can diffuse rapidly in the plane of their own monolayer, but cannot easily pass into the other monolayer. This lipid matrix provides the basic structure of the membrane. Whereas some protein molecules cover part of the membrane, particularly its outer surface, other protein strands penetrate the lipid layer, every here and there, and some of these strands are bunched together to form water-filled tubes or pores (Wallach and Zahler, 1966). These proteins are responsible for most of the membrane s functions, e.g. receiving and transduc-... 

Changes of lipid levels and compositions could alter the matrix in interactions with membrane proteins (e.g., ion channels), thereby influencing the protein configurations and functions. Moreover, alterations in membrane lipid components could also affect the microenvironments in cellular communication and cytosolic ion distribution. In this regard, glycolipids and gangliosides play important roles in the former while anionic lipids are the main players in the latter. 

Vesicles, which are self-assembled and closed phospholipid bilayers, have been widely studied as a typical model for the lipid matrix of the biological membrane and also as drug delivery systems [1,2], artificial cells [3], and for many other purposes. Among their various functions, the transport of proteins across the lipid membrane is an important issue. 

The above discussion indicates that a knowledge of the phase behaviour of individual molecular species of lipid comprising the matrix of the thylakoid membrane of higher plant chloroplasts can be informative of the factors governing the stability of the membrane. Perhaps the most important conclusion is that the presence of non-bilayer forming lipids is not simply required to facilitate the dynamic functions such as membrane fusion etc., but also to play a role in the creation of oligomeric functional complexes of the different membrane proteins. 

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