Practice Questions - Week 13 - Fatty Acid Metabolism SAQ's Flashcards
In cells, fatty acids are stored as triacylglycerols for energy reserves.
(a) What is the molecule to which fatty acids are esterified to form triacylglycerols?
(b) Define the logic behind cells storing fatty acids in esterified form.
(c) Describe three functions of triacylglycerols in mammals
Answer:
(a) Three fatty acids are esterified to glycerol.
(b) Triacylglycerols are uncharged and insoluble in water. They form lipid droplets within adipocytes, which do not contribute to the osmolarity of the cytosol in those cells, and do not require any water of hydration.
(c) Triacylglycerols provide mammals with
(1) stored fuel,
(2) insulation, and
(3) a source of metabolic water. In some animals, such as camels and desert
rats, the oxidation of stored lipids provides water; in hibernating animals,
oxidation of stored lipids generates heat to maintain body temperature
Describe the mechanism for moving acetyl-CoA produced in the mitochondrial matrix into the cytosol for fatty acid synthesis.
Acetyl-CoA in the mitochondrial matrix condenses with oxaloacetate to form citrate in a reaction catalysed by citrate synthase. Citrate moves out of the matrix via the citrate
transporter. Citrate in the cytosol is cleaved by citrate lyase, yielding acetyl-CoA and
oxaloacetate. To complete the cycle, oxaloacetate in the cytosol is reduced to malate,
which moves into the mitochondrial matrix on the malate-α−ketoglutarate transporter, and is converted to oxaloacetate in the matrix.
Describe how low glucose levels can decrease triacylglycerol synthesis in adipose tissue
Triacylglycerols are produced from fatty acyl-CoA and glycerol 3-phosphate.
The glycerol 3-phosphate is produced by the reduction of dihydroxyacetone phosphate, a glycolytic intermediate. Adipose tissue does not have glycerol kinase to form the
glycerol 3-phosphate from free glycerol. It must come from glucose
Draw the four basic steps in the oxidation of a saturated fatty acid (the β-oxidation pathway). Show structures, name enzymes, and indicate where any cofactors participate
Fatty acid synthesis and fatty acid breakdown occur by similar pathways. Describe, very briefly, four ways in which the synthetic and breakdown pathways differ.
Answer:
Fatty acid synthesis (in any order)
(1) employs NADPH as reducing agent;
(2) involves an acyl group bound to a protein, ACP;
(3) takes place in the cytosol of animals;
(4) involves the condensation of malonyl- and acetyl-groups;
(5) involves the formation of the D-β-hydroxyacyl derivative.
Fatty acid breakdown
(1) employs NAD+ as electron acceptor;
(2) involves acyl groups bound to coenzyme A;
(3) occurs in the mitochondrial matrix;
(4) does not involve malonyl-derivatives;
(5) involves the L-stereoisomer of the β-hydroxyacyl derivative.
Explain what is meant by the term the ‘starved-fed cycle’?
What are the stages?
Answer:
A starved-fed cycle is experienced in the hours after the evening meal, through night’s fast, culminating with breakfast the next morning. There are three stages: postabsorptive after the evening meal, early fasting during the night, and the refed state after breakfast.
What is the major fuel for resting muscle and what is the major fuel for muscle under strenuous work conditions?
Answer:
Fatty acids are the major fuel for resting muscle cells.
Glucose is the major fuel under strenuous exercise.
What are some of the primary metabolic fates of acetyl CoA?
Answer:
Acetyl CoA can enter the TCA cycle where it is oxidized to CO2.
It can also be exported as citrate to the cytosol where it can be converted into fatty acids.
It can be used in the synthesis of molecules, such as 3-hydroxy-3-methylglutaryl CoA, which are precursors of cholesterol and the steroids.
What are some of the recurring regulatory strategies employed by metabolic pathways?
Answer:
Metabolism is controlled by a number of common strategies including allosteric interactions with regulatory metabolites, covalent modification of enzymes, enzyme concentration control, compartmentalization, and metabolic specializations of different organs.
Compare in general terms the effects of adrenaline, glucagon, and insulin on glucose metabolism.
Answer:
Adrenaline and glucagon cause an increase in the blood glucose level.
Adrenaline acts when a higher than normal level of glucose is required; glucagon acts when the level is unusually low. Both stimulate gluconeogenesis and glycogen breakdown and decrease glycolysis and glycogen synthesis.
Insulin causes a decrease in blood glucose levels; it acts by increasing glycogen synthesis, glycolysis, and glucose uptake by cells as well as by decreasing glycogen breakdown.
