ANS 124: Milk Fat Synthesis

In non-ruminant mammary cells, what is the major carbon source of de novo fatty acid synthesis?

Glucose

Glucose is used to make what 2 things in the mammary gland?

Lactose Synthesis

Fatty Acid Synthesis

In non-ruminant FA Synthesis, what is glucose converted to when it is transported inside the mammary epithelial cell?

Glucose converted to Pyruvate

In non-ruminant FA Synthesis, Glucose is converted to Pyruvate where in the epithelial cell?

cytosol

In non-ruminant FA Synthesis, Glucose is converted to Pyruvate through what process?

Glycolysis

In non-ruminant FA Synthesis, where does Pyruvate go once it is made in the cytosol?

mitochondria

In non-ruminant FA Synthesis, when Pyruvate goes into the mitochondria, it is converted to what?

Acetyl CoA

In non-ruminant FA Synthesis, when Pyruvate is converted to Acetyl CoA in the mitochondria, can it leave the mitochondria?

No, Acetyl-CoA cannot leave the mitochondria.

In non-ruminant FA Synthesis, what happens to Acetyl-CoA in the mitochondria?

Acetyl-CoA is condensed with Oxaloacetate to create Citrate.

In non-ruminant FA Synthesis, Acetyl-CoA is condensed with Oxaloacetate to create Citrate via what overall process?

TCA Cycle

How many carbons does Citrate have?

6 carbons

In non-ruminant FA Synthesis, can Citrate leave the mitochondria?

Yes, citrate diffuses into the cytoplasm.

In non-ruminant FA Synthesis, what happens to Citrate in the cytoplasm?

ATP-driven cleavage of Citrate back to Oxaloacetate and Acetyl-CoA.

In non-ruminant FA Synthesis, when Citrate is cleaved, what happens to the Acetyl-CoA?

Acetyl CoA is now ready to enter the Malonyl CoA Pathway (FA synthesis).

In non-ruminant FA Synthesis, when Citrate is cleaved, what happens to the Oxaloacetate?

It enters the Malate Transhydrogenation Cycle.

In non-ruminant FA Synthesis, what does the Malate Transhydrogenation Cycle generate to help FA Synthesis?

it generates NADPH from NADP+, which are electron carriers that are used in FA synthesis.

In Ruminants, the capillary near the mammary gland contains what 4 things?

Chylomicron/Lipoproteins

Glucose

Glycerol

VFAs (Beta Hydroxy Butyrate & Acetate)

In ruminants, Chylomicrons deliver ___-___% of Triglycerides to the epithelial cells.

80-95%

In ruminants, VLDL deliver ___-___% of Triglycerides to the epithelial cells.

45-65%

The Triglycerides that Chylomicrons and VLDLs deliver are hydrolyzed to what 4 products to diffuse into the ECF?

Monoglycerides

Diglycerides

Free Fatty Acids

Glycerol

In ruminants, they do what process to a large proportion of dietary fats?

biohydrogenation (Unsaturated FA → Saturated FA)

In ruminants, can some Triglycerides reach the mammary gland without being hydrolyzed? How?

Yes, some can due to rumen bypass.

Describe the concentration of Glucose in the bloodstream of ruminants.

Low glucose

In ruminants, is Glucose broken down before going from blood to the ECF around epithelial cells?

No

In ruminants, is Glycerol broken down before going from blood to the ECF around epithelial cells?

No

In ruminants, are VFAs BHB & Acetate broken down before going from blood to the ECF around epithelial cells?

No

In Ruminant mammary cells, what are the 2 major carbon sources of de novo fatty acid synthesis?

Acetate & BHBA

In Ruminant mammary cells, is glucose a major carbon source for de novo fatty acid synthesis?

No, there is minimal use of glucose.

In Ruminant FA Synthesis, what is glucose converted to when it is transported inside the mammary epithelial cell? By what process?

Glucose converted to Pyruvate through glycolysis.

In Ruminant FA Synthesis, does Pyruvate still go in the mitochondria of the epithelial cell?

Yes

In Ruminant FA Synthesis, what 2 things can also go in the mitochondria of the epithelial cell?

Acetate and BHBA (VFAs) can enter mitochondria.

In Ruminant FA Synthesis, what happens to Acetate and BHBA when they enter the mitochondria?

they get converted to Acetyl-CoA and get shunted into the TCA cycle.

In Ruminant FA Synthesis, the TCA Cycle creates what 3 products that can diffuse out of the mitochondria into the cytosol?

Citrate

Isocitrate

α-Ketoglutarate

In Ruminant FA Synthesis, what happens to citrate, isocitrate and α-ketoglutarate in the cytoplasm?

Citrate gets converted to Isocitrate, which gets converted to α-Ketoglutarate.

In Ruminant FA Synthesis, what enzyme catalyzes Isocitrate to α-Ketoglutarate?

Isocitrate Dehydrogenase

In Ruminant FA Synthesis, what is also made when Isocitrate is converted to α-Ketoglutarate? How does that help FA synthesis?

NADPH is made from NADP+, which can transfer electrons in FA synthesis.

Is Isocitrate Dehydrogenase found in non-ruminants?

No, it is only found in ruminants.

In Ruminant FA Synthesis, is there a Malate Transhydrogenation Cycle?

No

In Ruminant FA Synthesis, since there's no Malate Transhydrogenation Cycle, would Oxaloacetate still be useful after Citrate is made?

No, since there would be no Malate Transhydrogenation Cycle for the Oxaloacetate to go to (like in non-ruminant FA synthesis).

In Ruminant FA Synthesis, besides Acetate & BHBA (VFAs) going to the mitochondria to be shunted into the TCA Cycle, where do they also go in the epithelial cell?

They also go to the cytoplasm to be converted to molecule that kickstart FA synthesis in ruminants.

Acetate is a precursor of what in FA Synthesis?

Acetyl-CoA

____________ is the starting block for making a fatty acid.

Acetyl-CoA

Acetyl-CoA is also used to make what other molecule in FA Synthesis?

Also used to make Malonyl CoA

Malonyl CoA is the precursor for what in FA Synthesis?

precursor for the carbons

In FA Synthesis, the fatty acid chain is elongated by how many carbons per round?

2 carbons per round

In the first step of FA synthesis, what is generated? From what?

Acetyl CoA → Malonyl CoA

What is the enzyme that catalyzes the first step of Acetyl CoA → Malonyl CoA in FA synthesis?

Acetyl CoA Carboxylase (ACC)

Give the full equation of the first step of FA Synthesis.

Acetyl-CoA + ATP + HCO3- → Malonyl CoA + ADP + Pi

What 2 minerals are also part of the first step of FA Synthesis?

Mg/Mn

Why is the Acetyl CoA Carboxylase (ACC) enzyme important in FA Synthesis?

It is the rate limiting enzyme for FA Synthesis (Acetyl CoA → Malonyl CoA is rate limiting step).

The transcription of the Acetyl CoA Carboxylase (ACC) gene is hormonally regulated when?

during lactation (lactogenesis)

around Lactogenesis I, fat synthesis starts, which is when ACC will be active to do FA synthesis.

What enzyme is responsible for FA chain elongation?

Fatty Acid Synthetase

What are the 5 steps of Fatty Acid Synthetase chain elongation (Malonyl-CoA Pathway)?

1. Acetyl CoA enters cycle to start a FA chain.

2. Condensation step joins Acetyl-CoA and Malonyl CoA.

3. First reduction Step

4. Dehydration Step

5. Second reduction Step

In the Malonyl-CoA Pathway, when adding another Malonyl CoA to the FA, it introduces how many carbons?

introduces 3 carbons

In the Malonyl-CoA Pathway, why does adding another Malonyl CoA only add 2 carbons to the chain even if it introduces 3 carbons?

One carbon is lost to CO2, so it only increases the chain length by n=2.

In Ruminants, what can enter the cycle of the Malonyl-CoA Pathway as a 4 carbon "primer"/extra precursor?

BHBA

Can BHBA enter the cycle of the Malonyl-CoA Pathway in nonruminants?

No, because they don't have VFAs in the bloodstream from the diet to go into the cycle.

In ruminants, if they have more supply of BHBA and acetate going into the cycle of the Malonyl-CoA Pathway, will the FA chains made be short or long?

More supply = chains not as long

For Ruminants, how does a Grain Diet affect propionate and acetate levels in the blood?

Increases Propionate

Decreases Acetate

For Ruminants, how does a Forage Diet affect acetate levels in the blood?

Increases Acetate

Since ruminants fed a forage diet have increase Acetate levels, how does that affect milk fat?

It increases the fat concentration of milk.

Fatty Acid Synthetase (FAS) is a large enzyme "____________".

complex

In the timeline from gestation to lactation, when does the activity of FAS and ACC increase?

increase around parturition

How are FAS and ACC activity regulated to increase around parturition?

the transcription of their genes are hormonally regulated

What enzyme prevents the synthesis of a FA chain from being too long?

Acylthioesterase

Acylthioesterase activity prevents the chain from being over how many carbons long?

>16 carbons

When does de novo FA Synthesis start on the timeline from gestation to lactation? When does it really amp up?

starts at Lactogenesis I, but really increases during Lactogenesis II

After FAs are synthesized, the synthesis of _______________ can occur.

triglycerides

What must happen to the FAs before triglyceride synthesis can occur?

Fatty acids need to be esterified first

In Ruminants, do Fatty acids need to be esterified first before triglyceride synthesis?

No, because in ruminants, the FAs are already esterified.

The synthesis of triglycerides occurs by what pathway?

Glycerol-3-Phosphate Pathway

What are the 3 steps of the Glycerol-3-Phosphate Pathway in the synthesis of Triglycerides?

1. Glycerol-3-PO4 + Fatty Acid

2. Lysophosphatidic Acid + Fatty Acid

3. Phosphatidic Acid + Fatty Acid = Triglyceride

The Glycerol used in triglyceride synthesis comes from what 2 places?

From blood

Synthesize glycerol as side product of glycolysis.

Are all Triglycerides made from scratch with fatty acids and glycerol?

Not all, monoglycerides from serum can have extra fatty acids added to the glycerol backbone.

How is the positioning of FAs on Triglycerides organized?

Smaller FAs are at the Sn3 position

Throughout lactation, milk can change in ________ and ____________.

yield & composition

Milk yield during lactation can be dependent on what 4 things?

Species dependent

Diet/Nutrition

Parity

Breastfeeding Frequency

When is the Peak Yield for cows during lactation?

at week 6

Why does it take 6 weeks for cows to reach Peak Yield during lactation?

Because the MG still has to adapt to a whole new state (from growth to milk synthesis). MG might still be making more cells to achieve peak lactation.

What happens to milk yield over time after Peak Yield is reached?

There is a Yield Decline.

What causes the Yield Decline during lactation?

mammary epithelial cell loss over time

What can cause milk yield to decline faster?

If there isn't enough milk removed, the rate of decline is faster.

Describe the levels of Fat, Protein, and Casein in milk during the first few weeks of lactation in a cow.

All 3 are high, and then they drop.

What are 2 reasons why levels of Fat, Protein, and Casein in milk during the first few weeks of lactation in a cow are high and then drop?

A lot of Protein (Ig) in colostrum is flushed out after first milk.

Dilution effect since water increases.

Overall, throughout a cow's lactation, what are the levels of Protein and Fat like?

they stay pretty static

Overall, throughout a cow's lactation, what happens to the level of Lactose in the milk?

Lactose decreases a little over time.

Lactose decreasing a little over time also contributes to what?

contributes to the yield decline in lactation

Is lactation a homeostatic process?

No, it is not.

Since lactation is not a homeostatic process, what type of process is it?

Homeorrhesis

Explain how lactation is a Homeorrhesis process.

Coordinated, physiologically mediated reallocation of biochemical resources to allow high levels of milk production.

Is Homeostasis maintained in lactaion?

Yes