W8 Lipid Metabolism

Question 1

available energy of food constituents

Answer 1

fat (adipose tissue): 555000 k J

protein (muscle): 102000

glycogen (muscle): 1920

glycogen (liver): 1120

glucose (extracellular fluid): 320

Question 2

primary sources of triacylglycerol

Answer 2

diet

de novo biosynthesis in liver

storage depots in adipocytes or adipose cells

fats insoluble > emulsified first with bile salts or complexed proteins to form lipoproteins

Question 3

how do hormones trigger the release of fatty acids from adipose tissues

Answer 3

synthesis of TAG and its deposition in adipose cells unlimited

mobilisation of stored fat (lipolysis) hormonally controlled by cAMP

mediated by Epi during stress and glucagon during fasting

other hormones regulate process under different conditions

Question 4

how are fats mobilised from dietary intake

Answer 4

alkaline pancreatic juice secreted into duodenum > raise pH of digestive mixture > hydrolysis of tracylglycerols by pancreatic lipase and non specific esterase

Question 5

how does bile salts help fat mobilisation during dietary intake

Answer 5

act as detergents to emulsify triglycerols and facilitate hydrolytic activity of lipases and esterase

fatty acids pass into epilethial cells > condensed with glycerol > form new triacylglycerols > aggregate with lipoproteins to form particles called chylomicrons

Question 6

difference between bile acids and bile salts

Answer 6

bile acids produced in liver from cholesterol > bile acid colic and chenodeoxycholic acid > found in unconjugated forms

COOH in bile acid reacts with either glycine or taurine > conjugated form of bile acids > form bile salts

Question 7

how do bile salts emulsify TAGs in the intestine

Answer 7

cholic acid ionises to give cognate bile salt

hydrophobic surface of bile salt associates with triacylglycerol > complexes aggregate to form a micelle

hydrophilic surface of bile salt faces outward > allow micelle to associate with pancreatic lipase/colipase

hydrolytic action of lipase/colipase frees fatty acids to associate in much smaller micelle > absorbed through intestinal mucosa

Question 8

generalised structure of plasma lipoprotein

Answer 8

hydrophobic inner core composed of cholesterol esters and tryacylglycerols surrounded by a hydrophilic surface formed by polar head groups of phospholipids and free cholesterol

Question 9

classification of lipoproteins

Answer 9

chylomicron: delivery of dietary fatty acids

VLDL: delivery of dietary and other fatty acids

IDL: delivery of dietary and other fatty acids

LDL: delivery of cholesterol

HDL: picking up excess cholesterol from cells for delivery back to liver

Question 10

what is the active site of lipoprotein lipase made up of

Answer 10

Ser, His and Asp

Question 11

first step of fatty acid oxidation

Answer 11

activation: fatty acid activated using coenzyme A and ATP > produce fatty acyl-CoA

Question 12

second step of fatty acid oxidation

Answer 12

fatty acyl-CoA cannot cross mitochondrial membrane but must be transported to mitochondrial matrix

carnitine (amino-oxy acid) undergoes ester-formation exchange reaction with fatty acyl-CoA > fatty acyl-carnitine ester that can move across membrane via facilitated diffusion

Question 13

third step of fatty acid oxidation

Answer 13

beta oxidation pathway

Question 14

3 main properties of the beta oxidation pathway

Answer 14

fatty acids must be degraded by oxidation at beta carbon followed by C(alpha)-C(beta) bond

this degradative process takes place in mitochondria

acetyl-CoA is released

Question 15

difference between beta oxidation in mitochondria and in peroxisomen

Answer 15

mitochondria: provides energy to organism

peroxisomen: responsible for shortening long chain fatty acids that are poor substrates for mitochondrial beta oxidation

Question 16

how does acyl-CoA synthetase activates fatty acid

Answer 16

fatty acid reacts with ATP > acyl adenylate (fatty acyl-AMP)

fatty acyl-AMP reacts with CoA > fatty acyl-CoA (activated form of fatty acids)

AMP is released

Question 17

how does the carnitine cycle help transport fatty acyl-CoA into the mitochondria

Answer 17

fatty acyl CoA converted to O-acylcarnitine using carnitine transferase I on outer mitochondrial membrane > release CoA > transported into inner membrane via translocase

acylcarnitine passed to carnitine transferase II on matrix side of inner membrane > transfers fatty acyl group back to CoA > reform fatty acyl CoA > leaving free carnitine > return across the membrane via translocate

Question 18

energy yield from oxidation of fatty acid

Answer 18

each round of oxidation produces one NADH, one FADH2 and one acetyl-CoA

oxidation of acetyl-CoA via citric cycle generates additional FADH2 and NADH > deoxidised through oxidative phosphorylation to form ATP

Question 19

where does oxidation of different fatty acids take place in

Answer 19

very long chain and long chain: peroxisome

medium chain and short chain: mitochondria

Question 20

how many atp produced per round of beta oxidation

Answer 20

each round > 2 carbon cleaved

1 NADH, 1 FADH2 and 1 acetyl CoA

acetyl coA goes into TCA > 3 NADH and 1 FADH2 and 1 GTP=1ATP

total atp = 17

Question 21

difference between beta oxidation in mitochondria and peroxisomes

Answer 21

mitochondria: ubiquinone is reduced in first step

peroxisomes: hydrogen peroxide produced

Question 22

how are fatty acids with odd number carbon chain oxidised

Answer 22

last step > 3C propionyl coA produced > carboxylation using propionyl-CoA carboxylase + biotin > D-methylmalonyl-CoA > L-methylmalonyl-CoA using methylmalonyl-CoA epimerase > succinyl coA using methylmalonyl-CoA mutate > enter TCA cycle to produce ATP

Question 23

definition of ketogenesis

Answer 23

process in which acetyl-CoA is converted to three metabolites: acetone, acetoacetate and beta hydroxybutyrate

ketone bodies synthesised primarily in liver but important sources of fuel and energy for many tissues

Question 24

how is beta hydroxybutyrate converted into acetyl coA

Answer 24

oxidised into acetoacetate by beta hydroxybutyrate dehydrogenase > acetoacetate coA transferase transfers coA from succinyl-CoA to acetoacetate > acetoacetyl-CoA > split into two molecules of acetyl CoA by thiolase

Question 25

what is the Lynen cycle

Answer 25

the formation of acetoacetate from two molecules of acetyl-CoA > formation of two HSCoA and 1 acetoacetate

Question 26

how is acetyl coA converted to malonyl coA to be the starting material for fatty acid synthesis

Answer 26

acetyl coA carboxylase (ACC) contains a biotin prosthetic group

ATP activates bicarbonate > carboxyphosphate > carboxyl group transferred to biotin > biotin transfers carboxy group to acetyl coA > form malonyl coA

Question 27

how is acetyl coA carboxylase regulated

Answer 27

activation by citrate: citrate promotes polymerisation of inactive ACC dimers into active polymeric form > allosterically activates ACC

inhibition by palmitoyl-CoA (final product of fatty acid synthesis): it seres as negative feedback inhibition