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Properties of Triacylglycerols

1. Hydrolysis• triacylglycerols undergo stepwise enzymatic

hydrolysis to finally liberate free FA and glycerols

• catalyzed by lipases• important for digestion of fat in the GIT and fat

mobilization from the adipose tissues

2. Saponification• hydrolysis by alkali to produce glycerol and

soaps

Triacylglycerol + 3 NaOH Glycerol + 3R-CooNa (soaps)

3. Rancidity• term used to represent the deterioration of fats

and oils resulting in an unpleasant taste• fats with UFA - more susceptible • occurs when fats are exposed to air, moisture,

light and bacteria

Metabolism of Lipids

Cholesterol, the most feared among the lipids, speaks:

“Consumed through diet and produced in the body;

Participate in innumerable cellular functions;

Implicated in several health complications;

And blamed I am, for no fault of mine!”

Cholesterol

• Best known steroid because of its association with atherosclerosis

• biochemically of significance because it is the precursor of a large # of equally important steroids

1. Sex hormones 5. Cardiac glycosides

2. Bile acids 6. Sterols

3. Vit. D 7. Sitosterol

4. Adrenal cortical hormones 8. Alkaloids

Cholesterol in Perspective

Cholesterol has many roles in health and disease, both in its own right, and as a precursor of a variety of biologically, important substances.

Cholesterol

GallstoneAtherosclerotic

plaqueLipoproteins

Membranes

Hormones

Adrenal / gonadsBile Acids

Acetate

U V lightVit. D

ATHEROSCLEROSIS

• Cholesterol penetrates into the T. interna, narrowing the vessels decreasing blood supply causing M I

• cholesterol must be packaged as part of a LIPOPROTEIN in order to be transported in blood

• its insolubility makes deposit troublesome especially in atherosclerotic plaques and gallstones

• A high level of blood cholesterol, especially that contained in LDL is a risk in atherosclerotic disease, much attention is being given to factors that lower cholesterol levels

a) DIET– substitute vegetable products for meat and

dairy products– consume more Polyunsaturated fatty acids

that saturated products– has a cholesterol lowering effects

b) Exercise HDL / LDL ratio which correlates negatively

with Atherosclerosis

KETOGENESIS AND THE ROLE OF KETONE BODIES IN ENERGY METABOLISM

acetoneKetone bodies acetoacetate

-hydroxybutyrate

acetone TRUE ketonesacetoacetate

-hydroxybutyrate-does not possess a keto (C=0) group

Ketone bodies are water soluble and energy yielding

liver - synthesis occur

mitochondrial matrix - where enzymes for ketone bodies synthesis are located

oxidation of FA Acetyl CoA

pyruvate

some AA precursor for ketone

bodies

Reactions of Ketogenesis

1. Two moles of acetyl CoA condense to form acetoacyl CoA

• reaction catalyzed by Thiolase (an enzyme involved in the final step of -oxidation)

• hence acetoacetate synthesis is regarded as the reversal of thiolase reaction of fatty acid oxidation

2. Acetoacyl CoA combines with another molecule of acetyl CoA to produce -methyl glutaryl CoA (HMC CoA)

• HMG CoA synthase, catalyzing the reaction, regulates the synthesis of ketone bodies

3. HMG CoA lyase cleaves HMG CoA to produce acetoacetate and acetyl CoA

4. Acetoacetate can undergo spontaneous decarboxylation to form acetone

5. Acetoacetate can be reduced by a dehydrogenase to -hydroxybutyrate

The C skeletons of some AA (ketogenic, leucine, lysine, phenylalanine, etc.) is degraded to acetoacetate or acyl CoA and to ketone bodies

Energy yield from oxidation of ketone bodies

1. Conversion of -hydroxybutyrate to acetoacetate yields an NADH molecule, yields 3 ATP molecules (by electron transport and oxidative phosphorylation)

2. Each mole of acetyl CoA that is formed yields 12 moles of ATP (via citric acid cycle, electron transport, and oxidative phosphorylation)

3. The activation reactions require 1 mole of ATP.

4. Therefore, oxidation of acetoacetate yields 24 moles (from 2 moles of acetyl CoA) - 1 mole

ATP (expended during activation) = 23 moles of ATP

oxidation of -hydroxybutyrate yields

3 moles of ATP (from one NADH molecule) + 24 moles of ATP (from 2 moles of acetyl CoA) - mole of ATP = 26 modes of ATP

During prolonged starvation ketone bodies are the major fuel source for the brain and other parts of the CNS

• Ability of the brain to utilize FA for energy is limited

• ketone bodies can meet 50-70% of the brain’s energy needs

In No individuals constant production of ketone bodies (liver)– concentration in the blood 1 mg/dL– excretion in urine is very low and

undetectable by routine tests (Rothera’s test)

Ketonemia rate of synthesis of ketone bodies exceeds the rate of utilization, their concentration in the blood

Ketonuria follows ketonemia, secretion of ketone bodies in the urine

Ketonemia ketosis (overall picture)

Ketonuria (+) acetone breath associated

with starvation and severe

uncontrolled DM

Glucagon stimulates ketogenesisInsulin inhibits

Ketogenic substances (promote ketone body formation)

1. Fatty acids

2. Amino acids (leucine, lysine, tyrosine)

Antiketogenic substances (inhibits ketone body formation)

1. Glucose

2. Glycerol

3. Glucogenic AA (glycine, alanine, serine, glutamate)

Acetoacetate strong acids - when in -hydroxybutyrate concentration in the blood

cause ACIDOSIS

Diabetic acidosis dangerous coma death if untreated

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