Slide Set 10: Milk Fat Biosynthesis Flashcards
1
Q
How much blood in the mammary tissue is needed for 1L of milk?
A
500L of blood
2
Q
Does rumen has enzymes?
A
No only bacteria
3
Q
What is the main feature of bacteria in the rumen?
A
Digest fibrous materials
- cellulose and hemi-cellulose
4
Q
Structural CHO is called
A
silage
5
Q
Non-structural CHO is called
A
concentrate
6
Q
What is the dietary CHO digested into by the bacteria of the rumen?
A
3 volatile fatty acids:
- butyrate
- propionate
- acetate
7
Q
What is the main source of energy for cows and humans?
A
for cows - butyrate
for humans - glucose
8
Q
How is acetate related to milk fat percentage?
A
Higher the acetate higher the milk fat percentage
9
Q
What is the main gluconeogenic volatile fatty acid?
A
propionate
10
Q
Why is propionate important?
A
- for lactose biosynthesis
- for development of the cell wall of the rumen
11
Q
Where is dietary fat digested?
A
in the small intestine both in humans and ruminants
12
Q
What is the composition of VLDL?
A
50% TG
22% Cholesterol
18% Phospholipids
9% Protein
13
Q
What is the composition of Chylomicrons?
A
83% TG
8% Cholesterol
7% Phospholipids
2% Protein
14
Q
Which lipoprotein is responsible for carrying lipids in humans and cows?
A
in humans it is chylomicrons
in cows it is VLDL
15
Q
Mammary gland can only synthesize _________ chain fatty acids
A
medium and short chain
C4-C14
16
Q
What are the sources of milk fatty acids?
A
- Blood plasma lipids (50%)
- De novo synthesis in the mammary gland (50%)
17
Q
What are the sources of blood plasma lipids?
A
- diet (80%)
- adipose tissue (20%)
18
Q
What are the pathways of fatty acid synthesis in the mammary gland?
A
- fatty acid oxidation (B-oxidation)
- Malonyl-CoA pathway
19
Q
Where is B-oxidation take place and what is the end product?
A
in the mitochondria
only short chain fatty acids
20
Q
Where does Malonyl-CoA pathway take place and what is the end product?
A
in the cytoplasm
elongation up to C16
21
Q
What is the main pathway of de novo milk fatty acid synthesis?
A
Malonyl-CoA pathway
22
Q
What are the two steps of Malonyl-CoA pathway?
A
- Formation of malonyl-CoA from Acetyl-CoA
2. Step wise elongation by addition of Malonyl-CoA
23
Q
What is the equation for formation of malonyl-CoA from Acetyl-CoA?
A
Acetyl CoA + ATP + CO2 —> Malonly-CoA + ADP + Pi
24
Q
What is the name of the enzyme required for the formation of malonyl-CoA from Acetyl-CoA?
A
Acetyl-CoA carboxylase
25
What is the equation for step wise elongation by addition of Malonyl-CoA?
Acetyl-Coa + 7Malonyl-CoA + 14 (NADPH + H+) ---> Palmitate + 7CO2 + 14NADPH + 8CoA + 6H20
26
What is the name of the enzyme required for the step wise elongation by addition of Malonyl-CoA?
Fatty acid synthetase
27
Explain the steps of the Malonyl-CoA pathway
* Acetyl CoA is carboxylated to malonyl CoA
* Malonyl CoA joins an acyl carrier protein
* Malonyl acyl complex attaches to acetyl CoA molecule to form β-hydroxybutyrate
* Hydrogenation of β-hydroxybutyrate produces butyryl Protein complex
* Addition of a second malonyl CoA increases chain length to C6
* More malonyl CoA molecules are added until C16
28
Complete elongation produces ____
C16
| no further elongation beyond that
29
How is are the fatty acids with less than C16 are synthesized in the mammary gland?
Premature termination of chain elongation by thioesterase II will produce medium & short chain fatty acids
30
What is the enzyme that is required for the premature termination of chain elongation?
thioesterase II
31
Explain the fatty acid biosynthesis in the mammary gland of monogastric mammals
- the glucose uptake of monogastric animals are much higher than ruminants
- glucose will be metabolized into pyruvate through glycolysis the pyruvate then enters the mitochondria and converted into acetyl-coA
- Acetyl-CoA cannot diffuse the membrane so the acetyl coA will be carried to the cytosol as citrate
- citrate will be cleaved into acetyl-coA and oxaloacetate in the cytosol
- this break down will take place by an enzyme called citrate lyase
- then Acetyl-CoA can be used for a primary source of fatty acid synthesis
- Acetyl-CoA will go into malate and will generate NADPH PENTOSE MONOPHOSTPHATE PATHWAY main source of NADPH for fatty acids
32
What is the main sources of Acetyl-CoA in ruminants?
acetate and B-HBA
33
What is the main source of Acetyl-CoA in non-ruminants?
produced in the mitochondria from pyruvate
34
What is the main source of reducing equivalent (NADPH) in ruminants?
conversion of isocitrate to a-ketogluterate
35
What is the main source of reducing equivalent (NADPH) in non-ruminants?
from oxaloactetate
36
What are the two sources of fatty acids for the biosynthesis of milk TG?
1. Blood lipids (dietary lipids and adipose tissue)
2. De novo synthesis (within the mammary
gland)
37
Where does milk TG biosynthesis take place?
In the SER
38
What are the two pathways for TG biosynthesis?
1. Glycerol 3 phosphate
| 2. Monoacylglycerol (monoglyceride)
39
What is the major pathway for milk TG biosynthesis?
Glycerol 3 phosphate pathway
40
What are the 3 sources of glycerol for milk TG biosynthesis?
1. Glucose via glycolytic pathway intermediates (70%)
2. Free glycerol from blood plasma (10%)
3. Plasma lipids, following lipolysis at the mammary cell capillary wall (20%)
41
Where does desaturation of stearic acid (C18:0) occur?
small intestine and mammary gland (more active in the mammary gland)
42
What is the main objective of desaturation of stearic acid?
the main objective is to maintain milk fat fluidity
43
What is the enzyme needed for desaturation of stearic acid? What is the end product?
desaturase
C18:0 to C18:1
44
What happens if the cow consumes a high fat diet?
High levels of fats in diets, depress intake and reduce ruminal digestibility
45
_________ fats are more detrimental to ruminal microbes than ______ fats (tallow)
Polyunsaturated fats (oils)
saturated fats
46
Explain the metabolism of dietary fat in the rumen
- dietary fat (mainly polyunsaturated) enters the rumen and gets hydrolyzed into polyunsaturated fatty acids (C18:3)
- Polyunsaturated fatty acids then go through isomerization and biohydrogenation into saturated fatty acids (C18:0) and partially unsaturated fatty acids
- then saturated fatty acids (C18:0) and partially unsaturated fatty acids are transported to the small intestine from the reticulo-rumen as mostly saturated FA
47
for the ruminal bacteria it is very import process to detoxify fatty acids as a defense mechanism. And it requires 2 steps:
1. isomerization conversion cis to trans. Most fatty acids in nature are in cis. trans only found in the rumen bc of the bacteria
2. biohydrogenation
48
How are unsaturated fatty acids converted to saturated fatty acids by the ruminal microbes?
1. Hydrolysis of triglycerides to fatty acids
2. Isomerization
3. Hydrogenation
49
What are the end product of fat metabolism in the rumen?
1. Saturated fatty acids (e.g. stearic acid, C18:0)
2. Partially saturated & isomerized fatty acids (e.g. cis-9, trans-11
C18:2)
3. Branched and odd number fatty acids of microbial origin
50
The extent to which dietary fat can alter milk fatty acid composition depends on:
o Fatty acid composition of dietary fat
o Degree of biohydrogenation in the rumen
o Influence of dietary fat on rumen microbes
51
What are the effects of high dietary fat on milk fatty acid composition?
Ø Decrease concentrations of short & medium chain fatty acids
Ø increase concentrations of long chain fatty acids
Ø increase concentration of PUFA (in some
conditions
Ø Decrease milk fat percentage (high levels)
52
What are the problems associated with milk high in PUFA?
* Milk with high levels of PUFA (C18:2 & C18:3) is more susceptible to autoxidation (short shelf life)
* Produces softer butter (oil off at low temperature)
* Requires separate collection and processing systems
* Modifications of processing facilities involved in production of dairy products (butter & cheese)
* A better alternative is to decrease the ratio of C16:0 to C18:1 without excessive increase in C18:2 and C18:3
53
What are health-promoting compounds found in milk?
* Conjugated linoleic acid (CLA)
* Butyric acid,
* Sphingomyelin
* B-carotene
* Omega-3 DHA
54
What are the two approaches to improve the quality of milk fatty acids?
1. Increase the concentration of polyunsaturated fatty acids of feed origin (Omega-3 fatty acid)
2. Increase fatty acid content of animal origin (e.g. CLA)
55
Why is CLA health-promoting?
* The only fatty acid shown to inhibit carcinogenesis
* Potent at very low concentration
* Ability to prevent diabetes
56
What are some nutritional strategies to increase CLA content in milk?
* Unsaturated fatty acid content of the diet
* High forage : concentrate ratio
* Source of dietary fat (substrate)
* Level of fat supplementation
* All factors that affect ruminal fermentation may affect CLA biosynthesis and trans fatty acid
57
How does dietary fat affect human milk?
Dietary fat intake does not affect total amount of fat in breast milk, however it influences the fatty acid composition
58
Foremilk vs Hindmilk
• Fore milk has a lower fat content than hind milk.
o As the infant continues to breastfeed over the next several minutes, the fat content increases.
• Hind milk is thought to facilitate satiety in the infant.
59
What are the PUFAs found in the human milk
DHA & AA
60
What does DHA concentration depend on?
DHA concentration varies according to diet (high with fish rich-diet).
61
What does AA concentration depend on?
AA concentration of breast milk is stable (derived from C18:2)
62
Cholesterol content of human milk vs formula based cow milk
Cholesterol content of breast milk is higher than formula-based cow milk
63
What are the health effects of human milk cholesterol?
Exposure to breast milk cholesterol may have long term benefits for cardiovascular health
64
What is the importance of DHA?
* Primary structural component of the human brain, cerebral cortex, skin, and retina
* Enhances the development of visual system (high levels in photoreceptors)
* Improve cognitive ability of infants
* Breast fed infants have higher levels of DHA than formula fed infants
* Breast fed infants accumulate DHA in the cortex, whereas formula fed infants maintain the same level found at birth
65
Breastmilk vs Formula Milk
* Baby formula was fortified with essential fatty acid precursors (linoleic and linolenic acids)
* In 2001, formula companies added DHA and AA to narrow the nutrient gap between breast milk and formula
* Added DHA was derived from algae and AA was extracted from soil fungi
* Human fatty acids are structurally different from plant FA (may cause digestive problems)
