Lipids:phospholipids, cholesterol, steroids

Biochem I, lecture 5Chapter 26.0-26.1 (exclude sphingolipids, p. 736-739), 26.2, 26.4 (exclude pregnenolone sections,

p. 752-754)Sept. 21 / 06

Both phospholipids and triacylglycerols are made from a common substrate

• Have addressed fatty acid synthesis and degradation but not yet that of the common carbon backbone of neutral lipids (which include triacylglycerols (TAGs), phospholipids (PLs), sphignolipids)

• Need to synthesize carbon backbone that links the FAs together: glycerol (3-C sugar alcohol)– Glycerol formed from glycolytic

intermediate dihydroxyacetone phosphate

– Glycerol sequentially acylated to form phosphatidate: common precursor for TAGs and PLs:

Glycolysis

1

2

3

(in animals)

p. 733

There are 2 possible metabolic fates for phosphatidate

• Acylate to a triacylglycerol– Remove phosphate at glycerol C3

by phosphatase– Attach acyl group on C3 using acyl

transferase– Synthesized primarily in liver,

transported to adipocytes for storage or muscles for energy conversion into ATP

• Synthesize a phospholipid

p. 734

Phospholipid synthesis requires substrate activation• Just like for catalysis of fatty acids which are activated by covalently

attaching c________• PLs synthesized from phosphatidate and an alcohol• Animals can activate either the phosphatidate or the alcoholOption 1: Activate diacylglycerolbackbone

• Activate phosphatidate with CTP to form CDP-diacylglycerol, then add an alcohol (like the amino acid serine) to form a phospholipid like phosphatidyl serine

CDP-diacylglycerol

Serine is just one of many alcohols (i.e., inositol as on p. 735) that can be conjugated to the C3 phosphate group of phosphatidate

p. 734

Phospholipid synthesis requires substrate activation

Option 2: Activate the alcohol– Take aminated ethanol– Phosphorylate it using ATP– Activate it by attaching cytidine diphosphate by CTP

usage– Conjugate the activated alcohol to diacylglycerol– Voilà: the phospholipid phosphatidyl ethanolamine

• Regardless of how they are formed, phospholipids can vary in structure and properties by varying– FA chains attached to C1 and C2– The alcohol connected to the phosphate

• A single mammalian cell can contain thousands of different PL species, each possessing distinct properties

Activated intermediate

p. 736

Cholesterol synthesis differs from that of neutral lipids

• Cholesterol – is a lipid and a steroid– modulates the fluidity of animal cell membranes– is the precursor of steroid hormones: progesterone,

testosterone, estradiol, cortisol• Recall that

– cholesterol is a lipid but is not linear like FAs– it is synthesized by a distinct pathway from 5 C units

(isoprene) versus FAs• 3 series of reactions to make cholesterol from isoprene

1. Synthesize isoprene in an activated form (isopentenyl pyrophosphate)

2. Condense 6 of these to make the branched hydrocarbon squalene

3. Cyclize squalene

Cholesterol is synthesized from acetyl CoA

Step 1: make isopentenyl PPi from acetyl CoA• Condense acetyl Co A with acetoacetyl Co A

(activated substrates) to mevalonate (committed step to cholesterol synthesis)

• Use 3 ATP molecules to add PPi to one end of mevalonate and precipitate decarboxylation and C=C double bond formation

Figs 26.7 and 26.8

NADPH NADP

Isoprene is synthesized in an activated form (with attached pyrophosphate

The activity of this reductase is the major control on cholesterol biosynthesis in the body

Count the Cs:4 + 2 = 6

Cholesterol is assembled from 6 isoprenesStep 2: form the C30 hydrocarbon squalene

• Assemble in series of condensation rxns in order C5 C10 C15 (x2) C30

• First isomerize activated isoprene to an allylic form (-C=CH-CH2):

• Transferase enzyme removes PPi from allylic substrates – Forms carbocation– Vulnerable to attack from electrons in double C=C bond of another molecule

of activated isopentenyl PPi– Cationic C centre is rapidly reduced to form C=C double bond

Pyrophosphate (PPi)

p. 740

More complex structures are formed by adding more isoprene

units

• Now have a C10 unit, repeat to form C15, condense 2 C15s to make squalene (C30)

Fig 26.9

C10C15

Cholesterol biosynthesis requires reducing power

• Squalene biosynthesis and cyclization, like mevalonate synthesis from acetyl Co A, require reducing power (NADPH)– Need electrons/H to reduce double bonds in

isoprene subunits– Recall that NADPH is reducing power that is

reserved for biosynthetic reactions

20 steps

Figs 26.11 and 26.12

Cholesterol serves many functions in animal cells

Bile salts(e.g.,

glycocholate)

Fig 26.23: steroid hormone biosynthetic pathway from cholesterol

• Cells throughout the body rely on cholesterol synthesized in liver and transported to them in lipoprotein vesicles (of low, intermediate or high density) where it serves as– Component of cell membranes– Precursor for other important

steroid molecules• Bile salts, steroid hormones,

vitamin D

– Bile salts: amphipathic detergents for solubilizing dietary lipids

• Glycocholate: major bile salt

Cholesterol is the precursor of all steroid hormones

• Remember that hormones initiate signal transduction cascades and thus change metabolism in target cells (e.g., to start breaking down stored triacylglycerols to free FAs in adipocytes and export them to muscle cells)

• Many different slight variations in hormone structure (though they are all similar: derived from cholesterol!) enable binding to different cell-surface receptors and thus activation of distinct cell targets

Vitamin D is also synthesized from cholesterol

Fig 26.28

Spontaneousisomerization

hydroxylations

• Synthesized not by an enzyme but by photolysis of one of the cholesterol rings by UV light

• Vitamin D is a steroid hormone that controls Ca and P metabolism, and bone mineralization

• a/k/a calcitriol: absence causes rickets• Hydroxylations in final conversion of vitamin D3 to vitamin D

performed by ubiquitous monooxygenase enzymes: cytochrome P450s

Cytochrome P450 enzymes are oxygenases• Very interesting class of enzymes

• Bind molecular oxygen (O2) with the help of a reductant, NADPH

• One O molecule used to hydroxylate (add OH) substrate (“R”) and the other to produce H2O

• Generic reaction:

• Contain a heme group, just like hemoglobin, and can thus bind O2

• NADPH contributes 1 e- to reduce the iron centre of hemoglobin from Fe3+ to Fe2+

• Acceptance of second e- cleaves bonds between O atoms in O2 and releases H2O

• Cytochrome P450s are important in detoxifying foreign (xenobiotic) substances in body

• Drugs are inactivated by these enzymes! Can also generate very carcinogenic compounds by metabolic activation (e.g., plant toxins, pigments)