Dr. M. Azhar ChishtiDept. Medical Biochemistry

Lecture Objective

1. To study the structure of physiologically important phospholipid classes- glycerophospholipids and sphingolipids. 2. To discuss the synthesis of phospholipids of clinical and physiological relevance and their degradation by phospholipases.

3. To study the role of phospholipids like phosphatidylinositol in signal transduction and membrane anchoring choline,

ethanolamine  and serine, cardiolipin and sphingomyelin etc.

Lecture Outline

1. Overview of phospholipids.

2. Structure of Glycerophospholipids and sphingolipids.

3. Synthesis of phospholipids: Phosphatidic acid, phosphatidyl-choline, ethanolamine   and serine, cardiolipin and sphingomyelin etc.

4. Degradation of phospholipids by phosphalipases.

5 . Role of phosphatidyl choline in lung surfactant

6. Role of phsophatidylinositol in signal transduction and membrane anchoring .

Polar compounds composed of alcohol attached by a بـ مربوط phosphodiester bridge to either diacylglycerol orرابطه

sphingosine.

Amphipathic ) ( للماء وكاره محب in nature, has a مترددhydrophilic head )phosphate + alcohol eg., serine, ethanolamine, and choline( and a long, hydrophobic tail )fatty acids or derivatives (.

In membranes, the hydrophobic portion is associated with the nonpolar portions such as glycolipids, proteins, and cholesterol.

The hydrohilic polar head extends outward, facing intracellular or extracellular aqueous environment )see Figure(.

Phospholipid

االسماء تشرح في شريحه

7

Membrane phospholipids also function as : 1- a reservoir حافظ for intracellular messengers, and, 2-for some proteins, phospholipids

serve as anchors ) ( مثبت .to cell membranes مرساه

Non-membrane-bound phospholipids يدخل لم الذي الفوسفولبدزوظيفه ) ( له حر الخلويه الجدر تركيب serve additional : فيfunctions in the body, for example, 1-as components of lung surfactant and 2-essential components of bile, where their البايل

تنظيف وظائف له الذي المكان detergent properties aid "in the هيsolubilization of cholesterol. الكلويسترول اخراج مثل

Glycerophospholipids : are formed from phosphatidic acid )PA( and an alcohol.

Serine + PA - phosphatidylserine )PS(�

Ethanolamine + PA - phosphatidylethanolamine )cephalin( �

Choline + PA - phosphatidylcholine )lecithin( PC�

Inositol + PA - phosphatidylinositol )PI(�

Glycerol + PA phosphatidylglycerol )PG(�

ارتباطات الفسفولبدز اوال : بالجليسرول

)Cardiolipin( اسممرض على للدالله يستخدم فسفولبدزالزهري

Two molecules of phosphatidic acid esterified through phosphate to an additional molecule of glycerol are called cardiolipin.

This is the only human glycerophospholipid that is antigenic.

cardiolipin is recognized by antibodies raised against Treponema the bacterium that causes syphylis الزهري.

[Cardiolipin is an important component of the inner mitochondrial membrane and bacterial membranes.]

Plasmalogens

When the fatty acid at carbon 1 of a glycerolphospholipid is replaced by an unsaturated alkyl group attached by an ether ايثر من rather than( رابطه to مهمه by an ester( linkage بدال

the core glycerol : molecule, a plasmalogen is produced.

Phosphatidalethanolamine is abundant بـ in nerve tissue is a غني plasmalogen.

Phosphatidalcholine )abundant in heart muscle( is the other quantitatively كميه significant ether lipid in mammals.

Platelet-activating factor (PAF)

PAF is an unusual ether ايضا ايثر برابطه مرتبطglycerophospholipid, with a saturated alkyl group قبل اللي عن - 1تختلف

مشبعه رابطه لديها الثاني in an ether link to carbon 1 and ان االختالف>>>2 - an acetyl residue )rather than a fatty acid( at carbon 2 of the

glycerol backbone.

PAF is synthesized and released by a variety of cell types thatbinds to surface receptors, triggering potent قوي thrombotic and

acute inflammatory leads to hypersensitivity & anaphylactic فعل ردود .reactions مناعيه

It causes platelets to aggregate and degranulate the neutrophils and alveolar macrophages to generate superoxide radicals that killed the bacteria.

PAF is one of the most potent bioactive molecules known, causing effects at concentrations as low as 10 -12 mol/L.

Sphingophospholipids or sphingomyelin

The backbone of sphingomyelin is the amino alcohol sphingosine, rather than glycerol.

A long-chain fatty acid is attached to the amino group of sphingosine through an amide linkage ( بعض في او استر كان قبل الليايثر ( producing a ceramide, which serve as a precursor of ,الحاالتglycolipids.

The alcohol group at carbon 1 of sphingosine is esterified : مهمه to phosphorylcholine, producing sphingomyelin, the only

significant sphingophospholipid in humans.

Sphingomyelin is an important componant of the myelin of nerve fibers )myelin sheath( that insulates and protects neuronal fibers of the central nervous system.

ثانيا : ارتباط الفسفولبدز بالسفينقو

Synthesis of Phospholipid

1. Glycerophospholipid )GP( synthesis involves بـاما تتم التصنيع : طرق either the1- donation of phosphatidic acid from CDP-diacylglycerol to an alcohol, or 2- the donation of the phosphomonoester of the alcohol from CDP-alcohol to 1,2-diacylglycerol. CDP is cytidine diphosphate.

2. In both cases, the CDP-bound structure is considered an "activated intermediate," and CMP is released as a side product of GP synthesis.

3. A key concept in phosphoglyceride synthesis, is activation either of diacylglycerol or the alcohol to be added by linkage with CDP. انظر الرسمه الصفحه القادمه

تريحك من صداع الراس هذا

الطريقه 1

الطريقه 2

في كل الحالتين يتم اخراج

CMP

الفرق بين الطريقتين فقط مكان وجود

CMP

4. The fatty acids esterifies to the glycerol alcohol groups can vary widely, contributing to the heterogeneity of this group of compounds.

5. Phospholipids are مهمه synthesized in the Smooth ER-- then transported to Golgi apparatus and then membranes organelles or plasma membrane, or are secreted by exocytosis الخلوي .االخراج

6. There are two classes of phospholipids: Those have either glycerol as a backbone or contain sphingosine. قبل من شرح كما

7. Both classes are found in membranes and play a role in the generation of lipid-signaling molecules.

A. Synthesis of phosphatidic acid (PA)

1. PA is the precursor االوليه of many المادهother phosphoglycerides. The steps in its synthesis from glycerol phosphate and two fatty acyl CoAs were in which PA is shown as a precursor of triacylglycerol.

2. All cells except mature erythrocytes can synthesize phospholipids, whereas triacylglycerol synthesis occurs only in liver, adipose tissue, lactating mammary glands, and intestinal mucosal cells.

B: Synthesis of phosphatidylethanolamine (PE) & phosphatidylcholine (PC)

• PC & PE are most abundant phospholipids in eukaryotic النواه حقيقه .cells المخلوقات

• The primary route of synthesis uses choline and ethanolamine obtained either from the diet or turnover تصنيع of the body's اعادهphospholipids.

• In the liver, PC also can be synthesized fromمهمهphosphatidylserine )PS( and PE.

1. Synthesis of PE and PC from preexisting choline and ethanolamine

These synthetic pathways involve the phosphorylation of choline or ethanolamine by انزيم kinases, followed by

conversion to the activated form, CDP-choline or CDP-ethanolamine تتوضح لكي الفسفولبدز لتصنيع الثانيه الطريقه الى . ارجع

Finally, choline-phosphate or ethanolamine-phosphate is transferred from the nucleotide )leaving CMP( to a molecule of diacylglycerol )see Figure 17.5(.

Significance of choline reutilization استعمال The :اعادهreutilization of choline is important because, whereas humans can synthesize choline de novo, the amount made is insufficient for our needs. Thus, choline is an essential dietary nutrient with an adequate

intake of 550 mg for men and 420 mg for women.

Role of PC in lung surfactant

1. The principal pathway for the synthesis of dipalmitoyl-phosphatidylcholine انظر للسيرفاكتينت االساسي التركيب في داخله2رقم )DPPC, or dipalmitoylecithin(. In DPPC, positions 1 and 2 on the

glycerol are occupied by palmitate. 2. DPPC, made and secreted by granular pneumocytes, is the

major lipid component of lung surfactant--the extracellular fluid layer lining the alveoli.

3. Surfactant serves to decrease the surface tension of this fluid layer, reducing the pressure needed to reinflate alveoli, thereby preventing alveolar collapse )atelectasis(.

4. Respiratory distress syndrome )RDS( in pre-term infants is associated with insufficient surfactant production, and is a significant cause of all neonatal deaths in western countries.

5. Lung maturity of the fetus can be gauged by determining the ratio of DPPC to sphingomyelin, usually written as the L )for lecithin(/S ratio, in amniotic fluid.

6. A ratio of 2 or above ) ( مهمه السليمه is evidence النسبه of maturity, because it reflects the major shift from دليلsphingomyelin to DPPC synthesis that occurs in the pneumocytes at about 32 weeks of gestation.

7. 1مهمه( العالج ( - Lung maturation can be accelerated by giving the mother glucocorticoids shortly تسرعbefore delivery. اخرى 2 طريقه -Administration of natural or synthetic surfactant )by intratracheal instillation تقطير

الهوائيه القصبه is also used in the prevention and )عبرtreatment of infant RDS.

8. Respiratory distress syndrome due to an insufficient amount of surfactant can also occur in adults whose surfactant producing pneumocytes have been damaged or destroyed, for example, as an adverse side effect of immuno-suppressive medication or chemotherapeutic drug use.

2: Synthesis of PC from phosphatidylserine (PS) in liver مهم :المكان

1. The liver requires a mechanism for producing PC, even when free choline levels are low, because it exports تصدر significant

amounts of PC in the bile and as a component of serum lipoproteins.2. To provide the needed PC, PS is decarboxylated الى to تحول

phosphatidylethanolamine )PE( by انزيم PS بواسطهdecarboxylase, an enzyme requiring pyridoxal phosphate as a cofactor.

3. PE then undergoes 3 methylation steps to produce PC, as illustrated in Figure 17.6. 4. S-adenosylmethionine )SAM( is the methyl group donor lead to S-adenosylhomocysteine

C. Phosphatidylserine (PS)

The primary pathway for synthesis of PS in mammalian tissues is provided by the base exchange reaction, in which the ethanolamine of PE is exchanged for free serine )see Figure 17.6(.

This reaction, although reversible, is used primarily to produce the PS required for membrane synthesis.

Phospholipid Lectuer#2

D. Phosphatidylinositol (PI)

PI is synthesized from free inositol and CDP-diacylglycerol as shown in Figure 17.5. ويمكن الفسفولبدز لصناعه االولى الطريق

القادمه الشريحه في كما الثانيه بالطريقه صناعته

PI is an unusual phospholipid in that it often contains stearic : مهمه acid on carbon 1 and arachidonic acid on carbon 2 of the glycerol.

جدا of arachidonic acid خازن PI, therefore, serves as a reservoirمهمهااللتهاب : ( عوامل الى تتحول اوليه ماده هو اسد االركانيود in ) معلومهmembranes and, thus, provides the substrate for prostaglandin عامل.synthesis when required التهاب

1. Role of PI in signal transmission االشاره across نقلmembranes:

a. The phosphorylation of membrane-bound phosphatidylinositol produces polyphosphoinositides, for example, phosphatidylinositol 4,5- bisphosphate )PIP2(.

b. The degradation of PIP2 by phospholipase C occurs in response to the binding of neurotransmitters, hormones, and growth factors to receptors on the cell membrane )Figure 17.8(.

c. The products of this degradation, inositol 1,4,5-trisphosphate )IP3( and diacylglycerol )DAG(, mediate the mobilization of intracellular calcium and the activation of PKC, which act synergistically to evoke يستدعي specific cellular responses. Signal transmission across the membrane is completed.

نقل االشاره عبر الغشاء الخلوي

PI1- يحدث فسفره للفينتج المركب اللي

بالصورهعند وجود اشاره من -2

الخارج كهرمون او غيره ينكسر المركب اللي

بالصوره ويعطي الكحول ) اينوزيتول ( مع الفسفور و يعطي

Diglycrolهذه العمليه تحفز -3 االشارات الخلويه

الصورة بالشريحه القادمه تسهل كل

الموضوع

مهمه وتسهل الموضوع وخصوصا

االسهم

2. Role of PI in membrane protein anchoring تدعيم

a. Specific proteins can be covalently تساهميه attached via a برابطهcarbohydrate bridge to membrane bound PI )Figure 17.9(.

b. Examples of such proteins include alkaline phosphatase )a digestive enzyme found on the surface of the small intestine that attacks organic phosphates(, and acetylcholine esterase )an enzyme of the postsynaptic membrane that degrades the neurotransmitter acetylcholine(.

c. Cell surface proteins bound to glycosyl phosphatidylinositol )GPI( are also found in a variety of parasitic protozoans الطفيليات )for example, Trypanosomes and Leishmania(.

d. Being attached to a membrane lipid rather than being an integral part of the membrane( allows GPI-anchored proteins rapid lateral mobility on the surface of the plasma membrane.

e. The protein can be cleaved from its anchor by the action of phospholipase C مهمه releasing diacylglycerol.

f. A deficiency in the synthesis of GPI in hematopoietic تكوين cells results in a hemolytic disease, paroxysmal nocturnal الدمhemoglobinuria.

E. Phosphatidylglycerol (PG) and cardiolipin

PG occurs in relatively large amounts in mitochondrial membranes and is a precursor of cardiolipin.

It is synthesized by a two-step reaction from CDPdiacylglycerol

and glycerol 3-phosphate.

Cardiolipin )diphosphatidylglycerol, is composed of two molecules of phosphatidic acid connected by a molecule of glycerol.

It is synthesized by the transfer of diacylglycerophosphate from CDP-diacylglycerol to a preexisting molecule of

phosphatidylglycerol.

F. Sphingomyelin

Sphingomyelin, a sphingosine-based phospholipid, is a مهمه major structural lipid in the membranes of nerve tissue. The synthesis of sphingomyelin is shown

in Figure, Briefly, palmitoyl CoA condenses with serine, as coenzyme A and the carboxyl group )as CO2( of serine are lost. This reaction, like the decarboxylation

reactions involving amino acids, requires pyridoxal phosphate )a derivative of

vitamin B6( as a coenzyme.

The product is reduced in an NADPH-requiring reaction to sphinganine, which is acylated at the amino group with one of a variety of long-chain fatty acids, and then desaturated to produce .a ceramide--the immediate precursor of sphingomyelinمهمه

A ceramide with a fatty acid thirty carbons long is aمهمهmajor component of skin, and regulates skin's water permeability.

Phosphorylcholine from phosphatidylcholine is transferred to the ceramide, producing sphingomyelin and diacylglycerol.

Sphingomyelin of the myelin sheath contains predominantly دائما

longer-chain fatty acids such as lignoceric acid and nervonic acid.

whereas gray matter of the brain has sphingomyelin that contains

primarily stearic acid.

IV. DEGRADATION OF PHOSPHOLIPIDS

The degradation of phosphoglycerides is performed by phospholipases جدا جدا جدا found in all tissues and مهمه

pancreatic juice.

A number of toxins and venoms سموم have phospholipase activity, and several pathogenic bacteria )Baccili( produce phospholipases that dissolve cell membranes and allow the spread of infection.

خاصه ) ( حاله مهمه Sphingomyelin is degraded by the lysosomal phospholipase, sphingomyelinase.

A. Degradation of phosphoglycerides a. Phospholipases hydrolyze يكسر the phosphodiester bonds of

phospho- glycerides, with each enzyme cleaving phospholipid at a specific site.

b. The major enzymes responsible for degrading phosphoglycerides are shown in Figure.

c. Removal of the fatty acid from carbon 1 or 2 of a phosphoglyceride produces a lysophosphoglyceride, which is the substrate for lysophospholipases.

d. Phospholipases release molecules that can serve as messengers )DAG and IP3(, or that are the substrates for synthesis of

messengers )arachidonic acid(.

e. Phospholipases are responsible not only for degrading مهمهphospholipids, but also for "remodeling اشكالهم .them "تغيير

f. For example, phospholipases A1 and A2 remove specific fatty acids from membrane-bound phospholipids; these can be replaced withalternative fatty acids using fatty acyl CoA transferase.

g. This mechanism is used as one way to create the unique lung surfactant, dipalmitoylphosphatidylcholine )see p. 202(, and to insure that carbon 2 of PI )and sometimes of PC( is bound to arachidonic acid.

B. Degradation of sphingomyelin

a. مرحلتين على الفصل 1يتم -Sphingomyelin is degraded by sphingomyelinase, a lysosomal enzyme that hydrolytically removes phosphorylcholine, leaving a ceramide 2. مهمه -The ceramide is, in turn, cleaved by ceramidase into sphingosine and a free fatty acid )Figure 17.12(.

b. The ceramide and sphingosine released by the degradation of sphingomyelin play a role as intracellular messengers.

c. Ceramides appear to be involved In the response to stress, and مهممهsphingosine inhibits protein kinase C.

d. Niemann-Pick disease )Types A and B( is an autosomalمهمهrecessive disease caused by the inability to degrade sphingomyelin. The deficient enzyme is sphingomyelinase-A type of phospholipase C.

e. In the severe infantile form )type A(, the liver and spleen are the primary sites of lipid deposits and are, therefore, tremendously

enlarged. The lipid consists primarily of the sphingomyelin that cannot be degraded )Figure 17.13(.

f. Infants with this disease have rapid, progressive neurodegeneration deposition of sphingomyelin in CNS,

and they die in early childhood.

g. A less severe variant )type B( causes little to no damage to neural tissue, but lungs, spleen, liver, and bone marrow are affected, resulting in a chronic form of the disease, with a life expectancy only to early adulthood.

h. Although Niemann-Pick disease occurs in all ethnic groups. both type A and B occur with greater frequency in the

Ashkenazi Jewish ) ( احسسسسسسسسسسسسن اشناكاز يهودpopulation than in the general population.

i. In the Ashkenazi Jewish population, the incidence of type A is 1:40,000 live births, and that of type B is 1:80,000.

j. The incidence of Niemann-Pick disease in the general population is less than 1:100,000.

Conclusions

1. Structure of Glycerophospholipids and sphingolipids.

3. Synthesis of phospholipids: Phosphatidic acid, PC, PE  and PC, cardiolipin and sphingomyelin etc.

4. Degradation of phospholipids by phosphalipases.

5 . Role of phosphatidyl choline in lung surfactant

6. Role of PI in signal transduction and membrane anchoring .

THANK YOU

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