Energy storage in cells

Josef Fontana

EC - 58

Overview of the lecture

• Introduction to the storage substances of human body

– Overview of storage compounds in the body

• Glycogen metabolism

– Structure of glycogen

– Synthesis and degradation of glycogen

– Phosphorylation and dephosphorylation as a regulatory

mechanism of the glycogen metabolism

• Synthesis of fatty acids and TAG

– Differences between synthesis and degradation of fatty acids

– How works the fatty acid synthase

– Elongation and desaturation of fatty acids

– Synthesis of TAG

Introduction to the storage

substances of human body

Overview of storage

compounds in the body

Overview of storage compounds

in the body • TAG

• Glycogen

• No storage protein

• TAG are excelent for energy storage - 1g of fat

has 6 times more energy than 1g of

hydrated glycogen

• Complete oxidation of 1g of FA = 38 kJ

• Complete oxidation of 1g of saccharides or

proteins only 17 kJ

Overview of storage compounds

in the body • 70 kg man has:

• 1) 420 000 kJ in TAG

• 2) 10 000 kJ in proteins (muscle)

• 3) 2 500 kJ in glykogen

• 4) 170 kJ in glucose

• Glycogen and glucose are sufficient to

supply the body one day, TAG many

weeks

Glycogen metabolism

Structure of glycogen

Glycogen

• Animal saccharide storage

• In liver (100g), skeletal muscle

(500g) and in small quantities in each

cell

• 1) liver glycogen: to maintain glycemia

• 2) muscle glykogen: for internal

muscle use

Glycogen structure

• Branched

homopolymer

• Most residues bound

by α 1→4 bonds

• Branching: α 1→6

bond

• These branches are

extended by α 1→4

bond

Glycogen has two ends

• Only on the non-reducing ends can

take place reactions (lengthening or

shortening)

• Reducing end is the one with the

hemiacetal hydroxy group - bound to

tyrosine in glycogenin

Glycogen metabolism

Synthesis and degradation of

glycogen

Glycogenesis (glycogen synthesis)

• Cytosol

• Glucose phosphorylation to Glc-6-P:

glucokinase in liver and hexokinase in muscle

• Isomeration of Glc-6-P → Glc-1-P:

phosphoglucomutase

• Glc-1-P reacts with UTP → UDP-Glc (activated

Glc, bond on C1): Glc-1-P uridylyltransferase

• UDP-Glc is bound to the non-reducing end of

glycogen: glycogen synthase

Glycogen synthase

• Binds UDP-Glc to the non-reducing

end of glycogen

• UDP is released

• Chain of glucose molecules

lengthens, until it reaches a certain

length and branching occurs

Branching enzyme

• Removes oligosaccharide (6-7 Glc

residues) from growing chain and adds it to

a hydroxy group on the C6 in Glc

• Forms α 1→ 6 bond

• These branches are extended by glycogen

synthase

• Branching enzyme = amylo-(1,4 – 1,6)-

transglycosylase

Regulation of glycogen synthesis

• Glycogen synthase is regulated by

phoshorylation:

– phosphorylation inactivates

– dephosphorylation activates

• Insulin activates

• Glucagon and adrenaline inhibit

Glycogenolysis

• Cytosol

• 1) Phosphorolytic cleavage

(inorganic phosphate is used):

glycogen phosphorylase – Glc-1-P

(Cori ester)

• 2) Isomeration of Glc-1-P to Glc-6-P:

phosphoglukomutase

Cutting branches off

• Degradation of glycogen stops at the 4th Glc

before the branching point

• Glucanotransferase (glycosyltransferase)

transfers three glucose residues from the 4-

residue glycogen branch to the main chain

• Only one glucose molecule remains (α 1→6

bond) – cleaved by debranching enzyme

(amylo-α1→6-glucosidase)

• Linear glycogen chain – glycogen

phosphorylase

Regulation of glycogenolysis

• Glycogen phosphorylase is

activated phosphorylated

• Phosphorylase kinase

• Insulin inhibits

• Counter-regulatory hormones

activate

Synthesis of fatty acids and TAG

Differences between synthesis

and degradation of fatty acids

Differences between synthesis and

degradation of fatty acids

• FA synthesis in cytosol, degradation in

matrix

• Intermediates of FA synthesis are bound

to ACP (acyl carrier protein),

intermediates of degradation bound to

CoA

• Enzymes of FA synthesis form one big

multienzyme complex - Synthase of FA,

degradation enzymes are free in matrix

Differences between synthesis and

degradation of fatty acids

• FA chain is extended by 2 carbon

atoms from AcCoA – activated

substrate is malonyl~CoA

• Reducing cofactor for synthesis is

NADPH, oxidising cofactors for

degradation are FAD and NAD+

Differences between synthesis and

degradation of fatty acids

• FA synthesis (on FA synthase) ends

with palmitate (C16)

• Further chain elongation and

formation of unsaturated acids

catalyse other enzymes

Synthesis of fatty acids and TAG

Synthesis of malonyl~CoA

Synthesis of malonyl~CoA

• Substrate for FA synthesis: AcCoA

• Carboxylation to malonyl-CoA

AcCoA + ATP + HCO3- → malonyl~CoA + ADP

+ Pi + H+

• AcCoA carboxylase (biotin – vitamin H or B7)

• Regulatory enzyme

• CO2 removed during condensation with

growing FA

Synthesis of fatty acids and TAG

How works the fatty acid

synthase

Mammalian fatty acid synthase

• Homodimer of 2 identical subunits (260 kDa)

• Each subunit has three domains connected by

moving regions:

• 1) domain 1 – substrates entry and condensing unit -

both transferases and condensing enzyme - CE

• 2) domain 2 – reduction unit – includes ACP, β-

ketoacylreduktase, dehydratase and enoylreductase

• 3) domain 3 – thioesterase cleaves palmitate

Mammalian fatty acid synthase

• Places where are bound

intermediates on FA synthase:

• 1) thiol group of cysteine (CE)

• 2) thiol group of

phosphopantetheine (bound to

serine in ACP)

Steps of FA synthesis

1. Synthesis of malonyl-CoA: acetyl-CoA

carboxylase

2. Reaction AcCoA + CE: acetyltransacylase

3. Reaction malonyl-CoA + ACP:

malonyltransacylase

4. Condensation reaction: condensing enzyme

Acetyl-CE + malonyl-ACP → acetoacetyl-ACP +

CE + CO2

Steps of FA synthesis

5. First reduction: β-ketoacylreductase

Acetoacetyl-ACP + NADPH + H+ → D-3-

hydroxybutyryl-ACP + NADP+

6. Dehydration: 3-hydroxyacyldehydratase

D-3-Hydroxybutyryl-ACP → crotonyl-ACP + H2O

7. Second reduction: enoylreductase

Crotonyl-ACP + NADPH + H+ → butyryl-ACP +

NADP+

FA synthase works as a dimer

• Condensation

between

malonyl-ACP

(one subunit)

and acetyl-CE

(second

subunit)

• New acyl

remains on

ACP

CO2

KONDENZACE-

S

C

CH3

O

SH

CE ACP

SHS

C

CH2

CO

O

O

CEACP

SHSH

CE ACP

SHS

C

CH2

O

C

CH3

O

CEACP

First reduction

D-3-Hydroxybutyryl-ACP

ACPS

CC

H2

O

CCH3

O

Acetoacetyl-ACP

REDUKCE

H+ + NADPH NADP+

ACPS

CC

H2

O

CCH3

OH H

Dehydration

H2O

ACPS

CC

O

CCH3

H

H

Krotonyl-ACP

DEHYDRATACE

D-3-Hydroxybutyryl-ACP

ACPS

CC

H2

O

CCH3

OH H

Second reduction

REDUKCE

H+ + NADPH NADP+

ACPS

CC

O

CCH3

H

H

Krotonyl-ACP

ACPS

CC

H2

O

C

H2

CH3

Butyryl-ACP

Process continues

• Change of subunits after one rotation

• Palmitate (C16) is an end product

• Thioesterase cleaves palmitate

from ACP - hydrolysis of the thioester

bond with phosphopantetheine

Palmitate synthesis requires

• 8 AcCoA, 14 NADPH a 7 ATP

• AcCoA produced in matrix – inner

mitochondrial membrane is

impermeable – transport via citrate

• 8 NADPH from the citrate transport

to cytosol and remaining 6 NADPH

in pentose cycle

Citrate as AcCoA bearer

• High level of citrate in matrix – transport

to cytosol – cleavage by ATP-citrate

lyase:

Citrate + ATP + HSCoA + H2O → AcCoA

+ ADP + Pi + OAA

• AcCoA and OAA have different fate in

cytosol

OAA returns to matrix

• Inner mitochondrial membrane is

impermeable to OAA

• Reduction of OAA to malate by cytosolic

malate dehydrogenase:

OAA + NADH + H+ → malate + NAD+

• Oxidative decarboxylation of malate by

NADP+-malate enzyme (malic enzyme):

Malate + NADP+ → Pyr + CO2 + NADPH

OAA returns to matrix

• Pyruvate transport to matrix –

carboxylation by pyruvate carboxylase:

Pyr + CO2 + ATP + H2O → OAA + ADP

+ Pi + 2 H+

• Summary equation:

NADP+ + NADH + ATP + H2O →

NADPH + NAD+ + ADP + Pi + H+

Regulation of FA synthesis

• Enough substrates (saccharides/AA) and energy

• AcCoA-carboxylase:

• 1) insulin activates

• 2) glucagon and epinephrine inhibit

• 3) citrate activates

• 4) inhibition by palmitoyl-CoA – feedback

inhibition

• 5) AMP inhibits

Synthesis of fatty acids and TAG

Elongation and desaturation of

fatty acids

Synthesis of other fatty acids

• Chain elongation – elongases

• Synthesis of unsaturated FA –

desaturation – desaturases

• ER membrane

Desaturation

• Mammals lack enzymes catalyzing

formation of the double bond further than

on C9

• New double bonds are always formed

between the existing double bond and a

carboxyl group

• Mammals can not synthesize linoleic (18 : 2

cis D9, D12) and linolenic (18 : 3 cis D9, D12,

D15) acid – both eare essential

Synthesis of fatty acids and TAG

Synthesis of TAG

Synthesis of TAG