Biochemistry 11: Lipid and Amino Acid Metabolism

Question 1

how are lipids digested?

Answer 1

mechanical digestion occurs in the mouth and stomach (minimal) – enter the small intestine mostly intact

chemical digestion occurs in the small intestine through bile, pancreatic lipase, colipase, and cholesterol esterase

these enzyme hydrolyse lipids into 2-monacylglyerol, free fatty acids, cholesterol

digested lipids may form micelles for absoprtion or may be absorbed directly

Question 2

how are lipids absorbed?

Answer 2

short chain fatty acids are absorbed across the intestine into the blood

long chain fatty acids are absorbed as micelles across the brush border into mucosa where they are converted to triacylglycerols and cholesteryl esters

assembled into chylomicroms for release into the lymphatic system via lacteals - re-enter bloodstream via thoracic duct

Question 3

how are lipids mobilized from adipocytes and lipoproteins?

Answer 3

lipids are mobilized from adipocytes by hormone-sensitive lipase activated by a fall in insulin characteristic of a postabsorptive state, cortisol, or epinephrine

hydrolyzes triacyglycerols –> fatty acids and glycerol

lipids are mobilized from lipoproteins and chylomicrons by lipoprotein lipase

releases fatty acids from the triacylglycerols in lipoproteins

Question 4

how are free fatty acids transported through the blood?

Answer 4

they are associated with albumin

Question 5

lipoproteins

Answer 5

biochemical assemblies whose primary purpose is to transport hydrophobic lipid molecules in water, as in blood plasma or other extracellular fluids

aggregates of apolipoproteins and lipids

examples include chylomicrons, VLDL, IDL, LDL, HDL

Question 6

chylomicrons

Answer 6

least dense type of lipoprotein (hightest fat/protein ratio)

the transport mechanism for dietary triacylglycerol molecules, cholesterol, cholesteryl esters from intestine to tissues

transported via the lymphatic system

Question 7

Very-Low-Density Lipoprotein (VLDL)

Answer 7

transports newly synthesized triacylglycerol molecules from the liver to peripheral tissues in the bloodstream

also contain fatty acids synthesized from excess glucose or retrieved from chylomicron remnants

Question 8

Intermediate-Density Lipoprotein (IDL)

Answer 8

a VLDL remnant in transition between triacylglycerol and cholesterol transport (therefore is an intermediate between VLDL and LDL/HDL)

formed from a triacylglycerol removed from VLDL

sometimes reabsorbed by liver or sometimes further processed in bloodstream (like picking up cholesteryl esters from HDL to form LDL)

Question 9

Low-Density Lipoprotein (LDL)

Answer 9

primarily transports cholesterol for use by tissues (either biosynthesis or membrane structure)

can also be used to make bile salts in the liver or by other tissues for steroid hormone synthesis

Question 10

High-Density Lipoprotein (HDL)

Answer 10

synthesized in the liver and intestines, released as dense, protein-rich particles into blood

has apolipoproteins for cholesterol recovery - excess cholesterol from blood vessels for excretion

involved in the reverse transport of cholesterol

Question 11

apolipoproteins

Answer 11

form the protein component of lipoproteins

control interactions between lipoproteins

Question 12

in what ways can cholesterol be obtained?

Answer 12

dietary sources of LDL or HDL

de novo synthesis in the liver driven by acetyl-CoA and ATP as well as insulin

Question 13

3-hydroxy-3-methylglutaryl (HMG)-CoA reductase

Answer 13

the key enzyme (rate-limiting) in cholesterol biosynthesis

synthesizes mevalonic acid in the smooth endoplasmic reticulum

Question 14

lecithin-cholesterol acyltransferase (LCAT)

Answer 14

enzyme in bloodstream activated by HDL apolipoproteins

catalyzes the formation of cholesteryl esters (cholesterol + fatty acid) for transport with HDL

Question 15

cholesteryl ester transfer protein (CETP)

Answer 15

catalyzes the transition of IDL to LDL by transferring the cholesteryl esters from HDL

Question 16

which essential fatty acids are important in maintaining membrane fluidity?

Answer 16

alpha-linolenic acid

linoleic acid

Question 17

how are fatty acids synthesized?

Answer 17

synthesized in the cytoplasm from acetyl-CoA transported out of the mitochondria after production by pyruvate dehydrogenase complex (crosses mito membrane as citrate)

five steps:

1. activation - attachment to CoA by acetyl-CoA carboxylase –> malonyl CoA
2. bond formation between malonyl CoA and an acyl carrier protein
3. reduction of a carbonyl to a hydroxy group
4. dehydration
5. reduction to a saturated fatty acid

repeated 8 times to form palmitic acid - the only fatty acid humans can make

Question 18

how are triacylglycerols synthesized?

Answer 18

formed by attaching three fatty acids (as fatty acyl-CoA) to glycerol 3-phosphate

3 fatty acids + glycerol

occurs primarily in the liver and somewhat in adipose tissue

in liver, triacylglycerols are sent to adipose tissue as VLDL

Question 19

how does fatty acid oxidation occur?

Answer 19

occurs in the mitochondria following transport by carnitine shuttle

carnitine acyltransferase I is the rate-limiting enzyme

fatty acid is activated by attachment to CoA by fatty-acyl-CoA synthetase

cyclic steps:

1. oxidation of fatty acid to form double bond
2. hydration of double bond to form hydroxyl group
3. oxidation of hydroxyl group to form a carbonyl
4. cleavage of carbonyl into a shorter acyl-CoA and acetyl-CoA

repeated until chain is shortened to 2 carbons, creating the last acetyl-CoA

Question 20

ketogenesis

Answer 20

the formation of ketone bodies during a prolonged starvation state due to excess acetyl-CoA in the liver

reversed by ketolysis to regenerate acetyl-CoA for use as an energy source in peripheral tissues

brain can derive up to 2/3 of its energy from ketone bodies during prolonged starvation

Question 21

protein catabolism

Answer 21

occurs primarily in the small intestine only under conditions of starvation

carbon skeletons are used for energy (glucogenic or ketogenic)

resulting amino groups are fed into urea cycle for excretion

Question 22

which amino acids are ketogenic?

Answer 22

lysine and leucine are only ketogenic

Question 23

what does the energy aquired from an amino acid depend on?

Answer 23

ability to be turned into glucose (glucogenic, ketogenic, both)

number of carbons it can donate to these processes (size of side chain)

Question 24

which amino acids are both ketogenic glucogenic?

Answer 24

phenylalanine, isoleucine, threonine, tryptophan, tyrosine