Biomembrane basic

Bio-membrane

Bibhudatta MohantyB.Tech Biotechnology VIT University

Introduction- Phospholipids Associate Non-covalently to

Form the Basic bi-layer Structure of Bio-membranes.

Bio membranes are large flexible sheets that serve as the boundaries of cells and their intracellular organelles and form the outer surfaces of some viruses.

Unlike the proteins, nucleic acids, and polysaccharides, membranes are assembled by the non-covalent association of their component building block

The primary building blocks of all bio-membranes are phospholipids, whose physical properties are responsible for the formation of the sheet like structure of membranes.

Phospholipids consist of two long-chain, non-polar fatty acyl groups linked (usually by an ester bond) to small, highly polar groups, including a phosphate.

In most phospholipids found in membranes, the phosphate group is esterified to a hydroxyl group on another hydrophilic compound.

In phosphatidylcholine, for example, choline is attached to the phosphate.

The negative charge on the phosphate as well as the charged or polar groups esterified to it can interact strongly with water .

The phosphate and its associated esterified group, the “head” group of a phospholipid, is hydrophilic, whereas the fatty acyl chains, the “tails,” are hydrophobic.

The amphipathic nature of phospholipids, which governs their interactions, is critical to the structure of bio-membranes.

When a suspension of phospholipids is mechanically dispersed in aqueous solution, the phospholipids aggregate into one of three forms: spherical Micelles and Liposomes and sheetlike, two-molecule-thick phospholipid bi-layers.

The type of structure formed by a pure phospholipid or a mixture of phospholipids depends on several factors, including the length of the fatty acyl chains, their degree of saturation, and temperature

In all three structures, the hydrophobic effect causes the fatty acyl chains to aggregate and exclude water molecules from the “core.”

Under suitable conditions, phospholipids of the composition present in cells spontaneously form symmetric phospholipid bi-layers .

Each phospholipid layer in this lamellar structure is called a leaflet .

The fatty acyl chains in each leaflet minimize contact with water by aligning themselves tightly together in the center of the bilayer, forming a hydrophobic core that is about 3 nm thick .

The close packing of these nonpolar tails is stabilized by the hydrophobic effect and van der Waals interactions between them. Ionic and hydrogen bonds stabilize the interaction of the phospholipid polar head groups with one another and with water.

Because of their hydrophobic core, bilayers are virtually impermeable to salts, sugars, and most other small hydrophilic molecules .

The phospholipid bilayer is the basic structural unit of nearly all biological membranes; thus, although they contain other molecules (e.g., cholesterol, glycolipids, proteins), biomembranes have a hydrophobic core that separates two aqueous solutions and acts as a permeability barrier

First, the hydrophobic core is an impermeable barrier that prevents the diffusion of water-soluble (hydrophilic) solutes across the membrane. Importantly, this simple barrier function is modulated by the presence of membrane proteins that mediate the transport of specific molecules across this otherwise impermeable bilayer.

The second property of the bilayer is its stability. The bilayer structure is maintained by hydrophobic and van der Waals interactions between the lipid chains. Even though the exterior aqueous environment can vary widely in ionic strength and pH, the bilayer has the strength to retain its characteristic architecture

Important properties of lipid bi-layer

A typical biomembrane is assembled from – 1) phosphoglycerides 2)Sphingolipids 3)steroids All three classes of lipids are amphipathic

molecules having a polar (hydrophilic) head group and hydrophobic tail.

Three Classes of Lipids Are Found in Biomembranes

I. Phosphoglycerides, the most abundant class of lipids in most membranes, are derivatives of glycerol 3-phosphate.

A typical phosphoglyceride molecule consists of a hydrophobic tail composed of two fatty acyl chains esterified to the two hydroxyl groups in glycerol phosphate and a polar head group attached to the phosphate group.

The two fatty acyl chains may differ in the number of carbons that they contain (commonly 16 or 18) and their degree of saturation (0, 1, or 2 double bonds). A phosphogyceride is classified according to the nature of its head group.

Phosphatidylcholines, the most abundant phospholipids in the plasma membrane.

All of these compounds are derived from sphingosine, an amino alcohol with a long hydrocarbon chain, and contain a long-chain fatty acid attached to the sphingosine amino group.

In sphingomyelin, the most abundant sphingolipid, phosphocholine is attached to the terminal hydroxyl group of sphingosine . Thus sphingomyelin is a phospholipid, and its overall structure is quite similar to that of phosphatidylcholine.

II. A second class of membrane lipid is the sphingolipids

Other sphingolipids are amphipathic glycolipids whose polar head groups are sugars. Glucosylcerebroside, the simplest glycosphingolipid, contains a single glucose unit attached to sphingosine. In the complex glycosphingolipids called gangliosides, one or two branched sugar chains containing sialic acid groups are attached to sphingosine. Glycolipids constitute 2– 10 percent of the total lipid in plasma membranes; they are most abundant in nervous tissue.

The basic structure of steroids is a four-ring hydrocarbon.

Cholesterol, the major steroidal constituent of animal tissues, has a hydroxyl substituent on one ring . Although cholesterol is almost entirely hydrocarbon in composition, it is amphipathic because its hydroxyl group can interact with water.

Cholesterol is especially abundant in the plasma membranes of mammalian cells but is absent from most prokaryotic cells.

As much as 30–50 percent of the lipids in plant plasma membranes consist of certain steroids unique to plants

Cholesterol and its derivatives constitute the third important class of membrane lipids, the steroids

Membrane proteins are defined by their location within or at the surface of a phospholipid bilayer. Although every biological membrane has the same basic bilayer structure, the proteins associated with a particular membrane are responsible for its distinctive activities .

The density and complement of proteins associated with biomembranes vary, depending on cell type and subcellular location. For example, the inner mitochondrial membrane is 76 percent protein; the myelin membrane, only 18 percent.

Biomembranes: Protein Components and Basic Functions

The lipid bilayer presents a unique two-dimensional hydrophobic environment for membrane proteins.

Some proteins are buried within the lipid-rich bilayer; other proteins are associated with the exoplasmic or cytosolic leaflet of the bilayer.

Protein domains on the extracellular surface of the plasma membrane generally bind to other molecules, including external signaling proteins, ions, and small metabolites (e.g., glucose, fatty acids), and to adhesion molecules on other cells or in the external environment

Domains within the plasma membrane, particularly those that form channels and pores, move molecules in and out of cells.

Domains lying along the cytosolic face of the plasma membrane have a wide range of functions, from anchoring cytoskeletal proteins to the membrane to triggering intracellular signaling pathways .

In many cases, the function of a membrane protein and the topology of its polypeptide chain in the membrane can be predicted on the basis of its homology with another, well characterized protein

Membrane proteins can be classified into three categories

1) integral, 2)lipid-anchored, 3) peripheral on the basis of the nature of the

membrane–protein interactions

Proteins Interact with Membranes in Three Different Ways

Integral membrane proteins, also called transmembrane proteins, span a phospholipid bilayer and are built of three segments.

The cytosolic and exoplasmic domains have hydrophilic exterior surfaces that interact with the aqueous solutions on the cytosolic and exoplasmic faces of the membrane.

These domains resemble other water-soluble proteins in their amino acid composition and structure. In contrast, the 3-nm-thick membrane-spanning domain contains many hydrophobic amino acids whose side chains protrude outward and interact with the hydrocarbon core of the phospholipid bilayer

Integral membrane protein

In all transmembrane proteins examined to date, the membrane-spanning domains consist of one or more helices or of multiple strands. In addition, most transmembrane proteins are glycosylated with a complex branched sugar group attached to one or several amino acid side chains.

Invariably these sugar chains are localized to the exoplasmic domains.

Lipid-anchored membrane proteins are bound covalently to one or more lipid molecules .

The hydrophobic carbon chain of the attached lipid is embedded in one leaflet of the membrane and anchors the protein to the membrane.

The polypeptide chain itself does not enter the phospholipid bilayer

Lipid anchored membrane protein

Peripheral membrane proteins do not interact with the hydrophobic core of the phospholipid bilayer. Instead they are usually bound to the membrane indirectly by interactions with integral membrane proteins or directly by interactions with lipid head groups. Peripheral proteins are localized to either the cytosolic or the exoplasmic face of the plasma membrane.

Peripheral membrane protein

The most common type of IMP is the transmembrane protein (TM), which spans the entire biological membrane. Single-pass membrane proteins cross the membrane only once, while multi-pass membrane proteins weave in and out, crossing several times.

Tran membrane protein divided into single pass and multipass membrane

Single pass TM proteins can be categorized as Type I, which are positioned such that their carboxyl-terminus is towards the cytosol, or Type II, which have their amino-terminus towards the cytosol.

Type III proteins have multiple transmembrane domains in a single polypeptide, while type IV consists of several different polypeptides assembled together in a channel through the membrane.

Type V proteins are anchored to the lipid bilayer through covalently linked lipids. Finally Type VI proteins have both a transmembrane domains and lipid anchors

Example of single pass membrane protein is Glycophorin A

Example of multi pass membrane protein is Bacteriorhodopsin.

Glycophorin A

Bacteriorhodopsin-

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