Cholesterol Biosynthesis

Question 1

Cholesterol

Answer 1

• vital component of cell membrane, precursor of steroid hormones and bile salts
• Deposition in arteries associated with CV disease and stroke.
• All carbon atoms for biosynthesis come from acetate in 3 stage process

Question 2

3 stages of cholesterol synthesis - summarised

Answer 2

• Stage 1: acetate is converted to C5 units – isoprene
• Stage 2: 6 Isoprene units fused to form a linear molecule – Squalene
• Stage 3: Linear molecule is circularised to produce cholesterol

Question 3

STAGE 1: Formation of Isoprene from acetyl-Co-A (first step)

Answer 3

• production of HMG-CoA, precursor for isoprene units
• Reaction is identical to that of ketone body formation
• Enzymes for cholesterol biosynthesis are cytosolic

Question 4

STAGE 1: Formation of Isoprene from acetyl-Co-A (second step)

Answer 4

• conversion of HMG-CoA to mevalonate
• Rate-limiting/controlling step of cholesterol biosynthesis
• Requires 2 molecules of NADPH – 4 electron reduction
• Hydroxal functional group required for next step

Question 5

STAGE 1: Formation of Isoprene from acetyl-Co-A (third step)

Answer 5

• phosphorylation of mevalonate
• ATP hydrolysis required
• Enzyme is Mevalonate-5-phosphotransferase

Question 6

STAGE 1: Formation of Isoprene from acetyl-Co-A (fourth step)

Answer 6

• Isoprenoid units are pyrophosphate molecules
• Need to convert phosphomevalonate to a pyrophosphate
• Requires hydrolysis of second ATP molecule

Question 7

STAGE 1: Formation of Isoprene from acetyl-Co-A (fifth step)

Answer 7

• 5-pyrophosphomevalonate – precursor for isopentenyl-PP
• Reaction is a decarboxylation requiring ATP hydrolysis
• Isopentenyl-PP acts as precursor for the second isoprenoid - dimethylallyl-PP

Question 8

STAGE 1: Formation of Isoprene from acetyl-Co-A (sixth step)

Answer 8

• Isopentenyl-PP converted to dimethylallyl-PP by Isopentenyl-PP isomerase
• Movement of C=C bond from C1 to C2
• Occurs by a protonation/deprotonation reaction (i.e. addition/removal of H+)

Question 9

STAGE 1: Formation of Isoprene from acetyl-Co-A (seventh step)

Answer 9

• Two different isoprenoids required for cholesterol

- Biosynthesis requires 2 dimethylallyl-PP and 4 isopentenyl-PP

Question 10

STAGE 2: Formation of Squalene (first step)

Answer 10

• Isoprenoid units are condensed together by Prenyltransferase in two reactions to form Farnesyl-PP.
• Reactions:
  1. Isopentenyl-PP to dimethylallyl-PP
  2. Isopentenyl-PP to geranyl-PP

Question 11

STAGE 2: Formation of Squalene (second step)

Answer 11

• Condensation of 2 X farnesyl-PP = Squalene
• Requires NADPH as reducing equivalent
• Enzyme is Squalene synthase
• Linear form of cholesterol

Question 12

STAGE 3: Circularisation of Squalene to Cholesterol (first step)

Answer 12

• First step epoxidation of squalene
• Requires molecular O2 and NADPH
• Formation of cyclic 3 atom ring

Question 13

STAGE 3: Circularisation of Squalene to Cholesterol (second step)

Answer 13

• conversion of epoxide group to a hydroxal group and linking of rings
• Enzyme – oxidosqualene cyclase
• Intermediate molecule – protosterol cation
• Conversion to lanosterol: 1) Removal of H+, formation of C=C bond, 2) Rearrangement of H atoms at tail

Question 14

STAGE 3: Circularisation of Squalene to Cholesterol (third step)

Answer 14

• Lanosterol converted to cholesterol – 19 steps
• Includes removal of 3 methyl groups, reduction of a double bond and migration of the other double bond
• Requires NADPH and O2 molecule

Question 15

5 classes of cholesterol

Answer 15

• Progestins
• Androgens
• Glucocorticoids
• Mineral glucocorticoids
• Oestrogens

Question 16

Functions of cholesterol

Answer 16

• Control metabolism
• Inflammation
• Immune function
• Sexual development
• Salt and water balance

Question 17

Cholesterol as bile acid precursor

Answer 17

• Modifications: reduce C5-C6 bond, hydroxylation C7/C12, conversion of C24 to COOH, epimerisation 3b-OH and conjugation
• First run C7 hydroxylation – rate limiting
• Enzyme is cholesterol 7a-hydroxylase (CYP7A1)
• Dietary fat absorption, cholesterol excretion

Question 18

How is cholesterol transported?

Answer 18

• in bloodstream as lipoprotein complex (cholesterol, TAGs, apolipoprotein).

Question 19

What enzyme converts cholesterol to cholesteryl ester for packaging?

Answer 19

• Acyl-CoA : cholesterol acyltransferase (ACAT)

Question 20

How is cholesterol packaged by liver?

Answer 20

• VLDLs (very low density lipoproteins)

Question 21

Cholesterol when released into bloodstream

Answer 21

• to be stored or utilised by adipocytes and muscle

Question 22

Cholesterol transport/utilisation

Answer 22

• Release of TAGs from VLDL via lipases on capillary surfaces results in IDL (intermediate-density lipoproteins) which are rich in cholesteryl esters
• IDLs then converted to LDLs (low-density lipoproteins) by the removal for further TAGs
• Peripheral tissues take up cholesterol from LDL.
• LDL/IDL return cholesterol to liver
• HDL ‘picks up’ and transports cholesterol from tissues/plasma from dying cells to liver

Question 23

LDL uptake by receptor-mediated endocytosis

Answer 23

• receptor functions to internalize LDL
• LDL is delivered to lysosomes where its degraded and its cholesterol is released for use in the synthesis of membranes, steroid hormones and bile acids

Question 24

When things go wrong with cholesterol

Answer 24

• Elevated blood cholesterol levels arise from genetic defects or high dietary intake
• Dietary cholesterol enters bloodstream as chylomicrons
• TAGS removed by peripheral tissue – remnants absorbed by liver

Question 25

What can we fix?

Answer 25

• Ingestion of anion exchange resins – bind bile salt and ↑ cholesterol excretion e.g. cholestyramine (Questran)
• Cholesterol reducing drugs (statins) – inhibitors of HMG-CoA reductase