Phosphogluconate pathway (pentose shunt) Classes of lipids. Phosphogluconate pathway (pentose shunt) Phosphogluconate pathway. G-6-P DHase.

Classes of lipids

1. fatty acids

2. triglycerides = glycerol + 3 f.a. 3. phosphoglycerides = gly.P + 2 f.a. 4. steroids

5. sphingolipids=sphingosine+X+f.a.

Phosphogluconate pathway

(pentose shunt) Main functions:

1. formation of NADP:H for lipid synthesis

2. formation of ribose-P Location in cell: enzymes found in

cytoplasm

Phosphogluconate pathway

(pentose shunt)

Regulation: glucose-6-P DHase catalyzes the first step NADP:H inhibits g-6-PDH activity

Phosphogluconate pathway

Substrate: glucose-6-P End-products:

depends on needs of cell

- in adipose cells, NADP:H is required for reducing lipid intermediates - in rapidly dividing cells, DNA syn. requires pentose-PO₄’s

O:H HC-OP

OH

=O HC-OP

O O

g-6-P

OH

OH

G-6-P DHase

NADP+

NADP:H + H+

H H

6-P-gluconolactone H

Lactonase

=O HC-OP=O

O

OH OH

H

O - C=O H -C-OH HO-C-H H -C-OH H -C-OH H -C-O-P=O H O

H:O:H . ._{. .}

6-phosphogluconate

6-phospho-gluconolactone

6-Phosphogluconate DHase

O-C=O H -C-OH HO-C-H H -C-OH H -C-OH H -C-O-P=O H O

H H -C-OH C=O H - C-OH H - C-OH H - C-O-P=O H O O=C=O

NADP:H + H+

NADP+

ribulose-5-P

g-6-P f-6-P f-1,6-diP 3-PG DHAP gly.P glu

PEP pyr

gly.P DHAP

pyr acetyl Co A

OAA malate OAA malate

citrate pyr

glu

isocitrate succinate

OAA

G succ.CoA malate glycogen

NADP:H

r-5-P c.citrate c.acetyl CoA

G-6-P DH deficiency

• isozymes catalyze G-6-P DH

reaction in red blood cells

• one isozyme is stable and

produces most of NADP:H

• 2nd isozyme is absent in many

people whose ancestors

survived in malarial areas

_{Regions of endemic malaria}

• NADP:H formed in red blood

cells by pentose shunt supplies H: ions for reducing oxidants and repairing damaged molecules

• high fever, or ingestion of drugs,

e.g. aspirin, causes extra NADP:H to be used in rbc

function of NADP:H in rbc

G-6-PDH deficient humans

• increased demands for NADP:H

occur after large doses of

quinones, aspirin, sulphonimides or high fever

• if unusually large demands for

NADP:H in rbc, then damaged proteins not repaired

Nitrates in diet

• NADP:H is used to form Hb-Fe++

from Hb-Fe+++ (met-Hb) in rbc

• in some individuals, the enzyme,

met-Hb reductase is inhibited by nitrates and the [functional Hb-Fe++] drops significantly, and anemia results

Fatty acid synthesis

Main function:

storage of acetyl residues for future energy needs Substrate:

cytoplasmic

acetyl CoA CH3 - C~S-CoA

O

Fatty acid synthesis

End-product: palmitate 16:0 (16 C : 0 =)

CH3 - (CH2) - COO

_

14 H H H H H H H H H H H H H H H O C-C-C-C-C-C-C-C-C-C-C-C C-C- C - C ~S-CoA H H H H H H H H H H H H H H H

16 1

Fatty acid synthesis

Regulation:

acetyl CoA carboxylase - catalyzes rate-limiting step - activated by cytopl. citrate

allosterically - inactivated by

active protein kinase

Fatty acid synthesis

Regulation:

Rate of triglyceride formation also regulates f.a. synthesis Location: cytoplasm of adipose

and liver cells

Citrate shuttle

- moves acetyl residues to cytoplasm

m. acetyl CoA

+ OAA citrate

citrate

+

cytopl. CoA

+

ATP

acetyl CoA + OAA + ADP + Pi citrate

acetyl~S CoA + OAA Citrate

H-C - C~S-CoA COO C=O H-C-OH COO _ _ O H H O- C=O H-C-H COO C HO H-C-H C O O -citrate lyase

C - C ~S CoA H H H O COO H-C-OH COO C=O c. m. acetyl CoA +

Citrate shuttle

1. acetyl CoA carboxylase

CH3-C~S-CoA _{O-C-CH2-C~S - CoA}

O O O

malonyl CoA N-carboxybiotin _biotin

ATP HCO

3

-- cytoplasmic citrate is required for activation of acetyl CoA carboxylase

ADP + Pi

2. Transfer of acetyl and malonyl residues to carrier proteins

CH₃-C ~ S-CoA

synthase-SH S-synthase CoA-SH

O - C - CH₂ - C ~S-CoA ACP-SH

O - C - CH₂ - C ~S - ACP

O _O

O O

O _O

-CH₃-C ~

CoA -SH

3. Condensation reaction

CH₃-C ~ S-synthase + O -C - CH₂ -C ~S -ACP

CO2 + CH3 - C - CH2 - C ~ S-ACP

O

O _O

O O

acetoacetyl ACP acetyl synthase

malonyl acyl carrier protein HS-synthase

4. Reduction reactions

CH₃ - C- CH₂ - C ~ S-ACP

O O

O OH

H

CH₃ - C- CH₂ - C ~ S-ACP

CH₃ - CH = CH - C ~ S-ACP O

NADP:H + H+

NADP+ CH₃ - C - C - C ~ S-ACP

O NADP:H + H+

HOH NADP+

..

H H H H

CH₃-CH₂-CH₂-C ~S-synthase O

O - C - CH₂ - C ~ S - ACP CH₃-CH₂ - CH₂ -C ~ S - ACP

CH₃ - CH₂ - CH₂ C

-O O

O

CH₂ - C

malonyl ACP

CO₂

~ S -ACP HS-synthase O || || || O

Second condensation

Reduction reactions

R - C- CH₂ - C ~ S-ACP

O O

O OH

H

R - C- CH₂ - C ~ S-ACP

R- CH = CH - C ~ S-ACP O

NADP:H + H+

NADP+ R - C - C - C ~ S-ACP

O NADP:H + H+

HOH NADP+

..

H H H H

palmityl CoA

O H H H H H H H H H H H H H H H || C-C-C-C-C-C-C-C-C-C-C-C C-C- C - C ~ S - CoA H H H H H H H H H H H H H H H

16 1

end product of fatty acid synthesis

Fatty acids in human cells

• fatty acid C:=

• palmitoleic 16:1

• oleic 18:1

• linoleic 18:2*

• linolenic 18:3*

• arachidonic 20:4

*essential fatty acids

Elongation of palmityl CoA

R - CH2 - C ~ S-CoA O

O R- C- CH₂ -C ~ S-CoA

R- CH -CH-C ~ S-ACP O

NADP:H + H+

R - C = C - C ~ S-ACP

O

NADP:H + H+

HOH NADP+

H H O O O-C-CH₂ C ~ S-CoA

O malonyl CoA

OH H

palmityl CoA

Elongation reaction

R - CH₂- CH₂ - C ~ S-ACP stearyl 18:0

O

Desaturation of fatty acids

(double bond formation)

- by enzymes miniature ETS systems

CH3 - (CH2)16 -C ~ S-CoA stearyl CoA

CH3 -(CH2)7 -CH=CH-(CH2)7 -C~ S-CoA

O

O

NADP:H NADP

H+ O=O

2H₂O

Triglyceride synthesis

Main function: storage of fatty acids for energy

Regulation: rate depends on availability of fatty acids and O=O Location: cytoplasm of adipose

cells (also liver)

Triglyceride synthesis

Substrates: fatty acyl CoA’s + glycerol PO₄ Endproduct: triglycerides

tri-palmitate tri-oleate

fatty acids NEVER free in

the cell

• fatty acids may be stored in

triglycerides, may be made into phosphoglycerides for cell membranes or put onto

cholesterol to form cholesterol esters

fatty acid

Triglyceride pathway

DHAP glycerol P

NAD:H NAD

CH₃ (CH₂)₁₄-C~S-CoA O CH₃ (CH₂)₁₄-C~S-CoA H₂-C-O-C-(CH₂)₁₄ -CH₃

O 2 CoA-SH H2-C-O-C-(CH2)14 -CH3

O H-O-H

H₂-C-O-P-O phosphatidate

O

O

P

Triglyceride pathway

H₂-C-O-C-(CH₂)₁₄ -CH₃ O

H-C-O-C-(CH₂)₁₄ -CH₃ O

H₂-C-O-P-O O

phosphatidate

H₂-C-O-C-(CH₂)₁₄ -CH₃ O

H-C-O-C-(CH₂)₁₄ -CH₃ O

H2-C-O-H

diacylglycerol H-O-H Pi

Triglyceride pathway

O

H-C-O-C-(CH₂)₁₄ -CH₃ O

H2-C-O-C-(CH2)14 -CH3

O

tripalmitate

H₂-C-O-C-(CH₂)₁₄ -CH₃ O

H-C-O-C-(CH₂)₁₄ -CH₃ O

H2-C-O-H

diacylglycerol

C oA-S~C-(CH₂)₁₄ -CH₃ CoA-SH

Hormonal control of lipid

synthesis

1. Glucagon and epinephrine act through cAMP to inhibit

acetyl CoA carboxylase.

glucagon

ATP

cAMP protein

kinase

inactive

G

active ac.CoA carboxylase

P

P P P

Hormonal control of lipid

synthesis

2. Insulin acts through other proteins (e.g. irs-1) to stimulate activity of:

insulin

binds to receptor

protein to cause irs-1 to form

irs-1 pyruvate DHase

citrate lyase

g-6-P f-6-P f-1,6-diP 3-PG DHAP gly.P glu

PEP pyr

gly.P DHAP

pyr acetyl Co A

OAA malate OAA malate

citrate pyr

glu

isocitrate succinate

OAA

G succ.CoA malate glycogen

NADP:H

r-5-P c.citrate c.acetyl CoA