Scarica Lipids - complete notes for exam e più Schemi e mappe concettuali in PDF di Biochimica solo su Docsity!
Lipids Lipids (or fats) are an heterogeneous group of molecules with insolubility property in polar solvents such as water (hydrophobicity) and solubility in a polar solvents (lipophilicity)
- do not form polymeric structures
- Small molecules
- Strong tendency to associate thru non-covalent hydrophobic interactions They tend to be sticky —> the tendency is to be together to reduce the amount of water molecules Biological role:
- energy reserve
- Structural elements of biological membranes
- Signaling Structure : Whatever there is a big ring of things is hydrophobic -—> polar Fatty acids (aliphatic monocarboxylic acids) —> are the main components of the various biological lipids
- weak acids (pKa = 4.5)
- The aliphatic chain contains 4 to 36 carbons atoms
- Joined by single bonds (saturated fatty acids) or double bonds (unsaturated fatty acids)
● When you have many single bonds the sat fatty acid is free. ● For double (almost always in cis configuration) you are not so free the shape is locked is difficult to change shape. ● We distinguish saturated and in saturated fatty acids based on the absence or presence of one or more C=C double bonds. Almost all the double bonds are in the cis configuration. —> 20 carbons with 5 double bonds
The esterification between the OH groups of glycerol and the carboxyl groups of fatty acids determines the loss of their polar character: it follows that triglycerides are non-polar, hydrophobic molecules and essentially insoluble in water. Therefore in the cells they constitute a separate phase in the form of microscopic oily droplets present in the aqueous cytoplasm. In vertebrates, some specialized cells (adipocytes) store large amounts of triglycerides as fat droplets that almost completely fill the cells. In animals, triglycerides also play an important role in thermal insulation of the body against a low temperatures, when they are accumulated in a subcutaneous position. Structural lipids Around each cell, lipids form a biological membrane which defines its size and keeps its content separate from the environment (semi-permeable barrier to the passage of polar molecules and ions). Membrane lipids Unlike reserve lipids which are neural; they are amphipathic —> hydrophobic tail and hydrophilic head
- hydrophobic interaction between tailed and hydrophilic interactions between heads and water direct the arrangement of these structure into bilateral sheets.
Classification : Glycerophospholipids (GP) Glycerol derivatives with two fatty acids esterified at C1 and C2 and a charge of very polar group joined to C3 by a phosphodiester bridge. From the name we notice that sugars are involved C1 + C2 + C3 with fatty acid + glycerol Most of the inner membrane of mitochondria is made of this lipid. Sphingolipids (SP) They derive from sphingosine, a long-chain amino alcohol in which the C1, C2 and C3 are analogous to those of glycerol. A fatty acid joined to C2 (amine bond) forms ceramide, the basic unit of sphingolipids.
Sphingolipids (SP) and blood group The blood group in humans is determined by the presence of specific oligosaccharides, exposed as polar heads on the cell surface and recognized as antigens by the immune system STEROLS (ST) The main sterol of eukaryotic membranes is cholesterol (has the role to fill the holes left due to present of the double bond and regulates the permeability of the membrane). Structurally, it has a steroid nucleus made up of four fused rings (cyclopentaphenanthrene). The steroid nucleus is relatively rigid (no rotation of the C-C bonds). Molecule = polar end (OH of C3) + hydrophobic end (steroid nucleus and the side chain of C17) + fatty acid at 16C
The polar region of cholesterol allows interaction with the phospholipid heads The hydrophobic moiety of cholesterol allows for association with phospholipid tails Cholesterol has other roles in overall metabolism:
- it is the starting substance for the synthesis of steroid hormones
- in the liver, it is used for the production of bile, a substance secreted in the duodenum which serves to emulsify food lipids and make them absorbable by the small intestine.
- Vitamin D Prenolic lipids They are synthesized from isopentyl pyrophosphate via the mevalonic acid pathway Despite the common precursor, phenol lipids and their derivatives are involved in very different biological events
Partial hydrogenation of unsaturated fatty acids leads to the formation of unsaturated fatty acids in the trans configuration. The high consumption of trans fat is correlated with an increased cardiovascular risk. Structure:
