LipidiIlipidi(dalgrecolipos=grasso)comprendononumerosesostanzeconcara7eris8cheeproprietàdiverse,accomunatedall’esserepra8camenteinsolubiliinacquaesolubiliinveceneisolven8organici(comeiltetraclorurodicarbonio,cloroformio,etere,ecc.).Dalpuntodivistafunzionale,possonoessereclassifica8inlipidididepositootrigliceridi(98%),chefungonodariservaenerge8caelipidicellulari(2%):fosfolipidi,glicolipidiecolesterolo,chehannoinvecefunzionestru5urale(inquantoprincipalicomponen8dellemembranecellulari).Unaterzacategoria,presenteintraccenell’organismo,svolgefunzionibiologichepiùspecializzatecomeormoni,mediatorilocali,secondimessaggeri,an9-ossidan9epigmen9perl’assorbimentodellaluce,ecc.(peres.ormonisteroidei,eicosanoidi,vitaminaE,vitaminaAore9nolo).Rappresentanoil15-20%circadelcorpodiunmammifero(10kginunindividuodi70kg).L’apportolipidicoraccomandatoammontaal30%dellecalorietotalineigiovaniediminuisceconilprogrediredell’etàfinoal20-25%.Oltreallaquan8tàèmoltoimportanteanchelaqualità:gliacidigrassisaturinondovrebberosuperareil10%dellecalorietotali,mentrelarimanentequotadovrebbeesserecopertadaimonoedaipolinsaturi.

Metabolismodeilipidi

Metabolismodeilipidi

Triacilglicerolo

Acidigrassia18atomidicarbonio(C18)

Ac.StearicoAc.OleicoAc.LinoleicoAc.Linolenico

Gliacidigrassipiùcomuni

Ideriva9dell’acidoarachidonico–glieicosanoidi

(ciclossigenasi)

Acidolinolenico(n-3oomega-3)(caneega5o)Acidoarachidonico(n-6oomega-6)(ga5o)Acidoarachidonico->eicosanoidi(mediatorilipidicipro-infiammatori,vasocostri5oriepro-aggregan9)EPAeDHA->resolvine(mediatorilipidician9-infiammatori,vasodilatatoriean9-aggregan9)

Acidigrassiessenzialipercaneega5o

Iprincipaliacidibiliarieiloroconiuga9

Taurina:amminoacidononstandard,essenzialeperlanutrizionedelga7o

Complesso fosfolipasi A2-micella

0.5–1µm

Membranebiologiche

ConceRchiave•Leproteineintegralidimembranacontengonounastru7uratransmembranalacuisuperficieèidrofobica.

•Leproteineperiferichedimembranainteragisconononcovalentementeconaltreproteineolipidialivellodellasuperficiedellamembrana.

•Leprincipalicara7eris8chebiochimico-funzionalidellemembranederivanodalleproteinedimembrana(trasporto,rece7oriale,eso-edendo-citosi,ecc).

Lamembranaplasma9ca

Figure 16.30 !

Action of phospholipase A2. X represents apolar head group. R1 and R2 are long hy-drophobic chains, making up much of thephospholipid molecule.

C

Phospholipase A2

H2O

C O

O H

1

Glycerophospholipid

O

O

C

O

O

3H2C H2CCH2 CH2CHCH

O

O

P O

O

X

Lysophosphoglyceride

OH

C

O

O

1 2

O

O

P O

O

X

32

R1 R2 R1R2

+

" Figure 16.31Structure of phospholipase A2 from cobravenom. Phospholipase A2 catalyzes the hydrolysis of phospholipids at lipid–water interfaces. The model shows how a phospholipid substrate (dimyristoyl phosphatidylethanolamine, space-fillingmodel) can fit into the active site of thewater-soluble enzyme. A calcium ion (purple) in the active site probably helpsbind the anionic head group. About half ofthe hydrophobic portion of the lipid wouldbe buried in the lipid aggregate. Mammalianphospholipases are structurally similar tothe venom enzyme. [PDB 1POB].

macromolecular particles known as lipoproteins. A lipoprotein has a hydrophobic corecontaining triacylglycerols and cholesteryl esters and a hydrophilic surface consisting ofa layer of amphipathic molecules such as cholesterol, phospholipids, and proteins(Figure 16.32).

The largest lipoproteins are chylomicrons that deliver triacylglycerols and choles-terol from the intestine via the lymph and blood to tissues such as muscle (for oxida-tion) and adipose tissue (for storage) (Figure 16.33). Chylomicrons are present in bloodonly after a meal. The cholesterol-rich remnants of chylomicrons—having lost most oftheir triacylglycerol—deliver cholesterol to the liver. Liver cells are responsible for syn-thesizing most of the newly synthesized cholesterol that enters the bloodstream but al-most all cell types make cholesterol for internal use. Lipoproteins deliver both dietaryand liver-derived cholesterol to the rest of the body’s cells. Cholesterol biosynthesis isregulated by hormones and by the levels of cholesterol in the blood.

Blood plasma contains several other types of lipoproteins. They are classified ac-cording to their relative densities and types of lipid (Table 16.1). Since proteins aremore dense than lipids, the greater the protein content of a lipoprotein, the greater itsdensity. Very low density lipoproteins (VLDLs) consist of approximately 98% lipid andonly 2% protein. VLDLs are formed in the liver and carry lipids synthesized in the liver,or not needed by the liver, to other tissues such as adipose tissue. Lipases within capil-laries of muscle and adipose tissue degrade VLDLs and chylomicrons. When VLDLsgive up triacylglycerols to tissue cells their lipid content decreases and their remnants

BOX 16.6 EXTRA VIRGIN OLIVE OIL

Olive oil contains mostly triacylglycerols. If it has been pro-duced by crushing olives with no additional chemical treat-ment, then it is called virgin olive oil according to the Inter-national Olive Oil Council (IOOC).

The quality of olive oil is often determined by the pres-ence of free fatty acids that form when triacylglycerols breakdown during production. Virgin olive oil should have lessthan 2% free fatty acids (acidity) and extra virgin olive oil hasless than 0.8% free fatty acids (acidity).

! Extra virgin olive oil. Extra virgin olive oil has less than 0.8% freefatty acids. http://www.examiner.com/fountain-of-youth-in-atlanta/extra-virgin-olive-oil-benefits

506 CHAPTER 16 Lipid Metabolism

Tossicologiaalivellomolecolare:ilvelenodelcobra

Struttura tridimensionale della fosfolipasi A2 dal veleno di cobra, con�un suo substrato (un glicerofosfolipide)

Figure 16.30 !

Action of phospholipase A2. X represents apolar head group. R1 and R2 are long hy-drophobic chains, making up much of thephospholipid molecule.

C

Phospholipase A2

H2O

C O

O H

1

Glycerophospholipid

O

O

C

O

O

3H2C H2CCH2 CH2CHCH

O

O

P O

O

X

Lysophosphoglyceride

OH

C

O

O

1 2

O

O

P O

O

X

32

R1 R2 R1R2

+

" Figure 16.31Structure of phospholipase A2 from cobravenom. Phospholipase A2 catalyzes the hydrolysis of phospholipids at lipid–water interfaces. The model shows how a phospholipid substrate (dimyristoyl phosphatidylethanolamine, space-fillingmodel) can fit into the active site of thewater-soluble enzyme. A calcium ion (purple) in the active site probably helpsbind the anionic head group. About half ofthe hydrophobic portion of the lipid wouldbe buried in the lipid aggregate. Mammalianphospholipases are structurally similar tothe venom enzyme. [PDB 1POB].

macromolecular particles known as lipoproteins. A lipoprotein has a hydrophobic corecontaining triacylglycerols and cholesteryl esters and a hydrophilic surface consisting ofa layer of amphipathic molecules such as cholesterol, phospholipids, and proteins(Figure 16.32).

The largest lipoproteins are chylomicrons that deliver triacylglycerols and choles-terol from the intestine via the lymph and blood to tissues such as muscle (for oxida-tion) and adipose tissue (for storage) (Figure 16.33). Chylomicrons are present in bloodonly after a meal. The cholesterol-rich remnants of chylomicrons—having lost most oftheir triacylglycerol—deliver cholesterol to the liver. Liver cells are responsible for syn-thesizing most of the newly synthesized cholesterol that enters the bloodstream but al-most all cell types make cholesterol for internal use. Lipoproteins deliver both dietaryand liver-derived cholesterol to the rest of the body’s cells. Cholesterol biosynthesis isregulated by hormones and by the levels of cholesterol in the blood.

Blood plasma contains several other types of lipoproteins. They are classified ac-cording to their relative densities and types of lipid (Table 16.1). Since proteins aremore dense than lipids, the greater the protein content of a lipoprotein, the greater itsdensity. Very low density lipoproteins (VLDLs) consist of approximately 98% lipid andonly 2% protein. VLDLs are formed in the liver and carry lipids synthesized in the liver,or not needed by the liver, to other tissues such as adipose tissue. Lipases within capil-laries of muscle and adipose tissue degrade VLDLs and chylomicrons. When VLDLsgive up triacylglycerols to tissue cells their lipid content decreases and their remnants

BOX 16.6 EXTRA VIRGIN OLIVE OIL

Olive oil contains mostly triacylglycerols. If it has been pro-duced by crushing olives with no additional chemical treat-ment, then it is called virgin olive oil according to the Inter-national Olive Oil Council (IOOC).

The quality of olive oil is often determined by the pres-ence of free fatty acids that form when triacylglycerols breakdown during production. Virgin olive oil should have lessthan 2% free fatty acids (acidity) and extra virgin olive oil hasless than 0.8% free fatty acids (acidity).

! Extra virgin olive oil. Extra virgin olive oil has less than 0.8% freefatty acids. http://www.examiner.com/fountain-of-youth-in-atlanta/extra-virgin-olive-oil-benefits

506 CHAPTER 16 Lipid Metabolism

TIPIdiTRASPORTOa5raversolamembrana

TrasportoFacilitato:iltrasportodelglucosio