Sterol function membranes

The major dietary lipids for humans are animal and plant triacylglycerols, sterols, and membrane phospholipids. The process of lipid metabolism fashions and degrades the lipid stores and produces the structural and functional lipids characteristic of individual tissues. For example, the evolution of a highly organized nervous system has depended on the natural selection of specific enzymes to synthesize and degrade (turn over) the lipids of the brain and central nervous system. 

Nes, D.W., Janssen, G.G., Crumley, F.G., Kalinowska, M., AMhisa, T. (1993) The structural requirements of sterols for membrane function in Saccharomyces cerevisiae. Arch. Biochem. Bioph., 300, 724-733. 

A comparison of sterol formation in untreated barley powdery mildew with that in rice blast exposes as the major difference a variation in the structure of the functional membrane sterol. In the mildew sterol the A double bond of ergosterol is absent and isomerisation of the a24(28) double bond into the 22-position of the sidechain has not occurred. Nevertheless, the mildew sterol is a quasi-planar functional membrane component. 

Djerassi et al. have found a group of characteristic sterols in marine organisms which have side chains ranging from 0 to 6 carbon atoms. They occur as minor components in various species of Porifera and Coelenterata [78] and may be formed by in vivo auto-oxidation, or be of dietary origin. Accumulation of auto-oxidation products may be more prevalent in marine invertebrates than in mammals, since many marine organisms are known to store excretion products. The side chains are apparently too short for the sterols to be functional membrane constituents, but if these sterols are shown to arise by in vivo or environmental auto-oxidation in the oceans, it will have important imphcations in marine biochemistry and ecology. 

The polyenes have an affinity for sterol-containing membranes, insert into the membranes, and disrupt membrane functions. 

The main occurrence of free sterols Is In the cytoplasmic membrane, where they Interact with other lipids and proteins. Two modes of action for sterols In membranes are proposed. One Is the so called bulk menfl>rane function, l.e., the Interaction with phospholipids and the spatial separation of these charged molecules [13]. The other role Is a cofactor function for the Incorporation of unsaturated fatty acids Into lipids [14]. 

In milk fat, cholesterol is associated with Hpoproteins in the milk fat globule. It is also a component of animal membranes and controls rigidity and permeabihty of the membranes. Cholesterol has interesting surface properties and can occur in Hquid crystalline forms. Plants contain sterols such as P-sitosterol [83-46-5] (4b) or stigmasterol [83-48-7] (4c). Their functions in plant metaboHsm are not yet well understood. Analysis of sterols has proven useful for detection of adulteration of edible fats (9). 

In general, the sterols perform a structural function, for example as components of the lipid layers of membranes. The Cis, C19 and C21 steroids mainly perform an endocrine function. In other words they are hormones. The bile salts (C24-steroids) fulfil a functional role in digestion in animals. 

Sterols are derivatives of terpenes, within the larger chemical family of terpenoids. Such compounds are widespread and numerous in eukaryotes. Although in many cases their function is not clear, others are known to have an essential role in growth, development, metabolism and the integrity of cells. Sterols are localised in cell membranes, conferring stability and controlling permeability. 

In fungi, ergosterol, synthesised from acetyl-CoA, is the major sterol (Figure 4.2 Mercer2) and has an essential role in the maintenance of membrane function such that a reduction in its availability disrupts membrane integrity. 

The action of propamocarb is related to membrane function, causing efflux of cell constituents. Leakage ceases after the development of mycelium and can be inhibited by the addition of sterols. 

Binds to sterols in fungus cell membrane, resulting in an inability to function as a selective barrier and a subsequent loss of essential cellular components... 

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