Milk Biosynthesis...Pentose Phosphate Pathway Summary: Ruminants vs Monogastrics Monogastrics:...

Milk BiosynthesisPART 2: ENERGY

Key Enzymes(from all biosynthesis lectures)FDPase = fructose diphosphatase

Citrate lyase

Isocitrate dehydrogenase

Fatty acid synthetase

Acetyl CoA carboxylase

Fatty acyl deacylase – thioesterase II

Lipoprotein lipase

NAD, NADHNADH

◦ Nicotinamide adenine dinucleotide

◦ Reduced form

◦ Pyridine nucleotide that functions as an oxidative cofactor in eukaryotic cells

◦ Plays a key role in the production of energy through redox reactions

NAD

◦ Oxidized form

◦ Serves as a cofactor for dehydrogenases, reductases, and hydroxylases

◦ Major carrier of H+ and e- in major metabolic pathways like glycolysis, the TCA cycle, and fatty acid synthesis

NADH important in glycolysis and Citric Acid cycle

NADPH is used in cytosol for fatty acid synthesis

Energy and Carbon Sources

Mammary Gland Primary Energy Source

Whole Body Primary Energy Source

C-SourceLactose

Blood GlucoseSource

C-SourceFatty Acids

Ruminant Glucose Acetate Glucose Propionate Acetate and butyrate

Monogastric Glucose Glucose Glucose Diet Glucose

GlycolysisEmbden Meyerhof PathwayMetabolic pathway used to begin breakdown to glucose

Purpose: TO PRODUCE PYRUVATE AND ATP

Pentose Phosphate PathwayAlternative pathway of glucose metabolism

Generates pentoses, particularly ribose for use in nucleic acid synthesis

Produces NADPH

◦ Tissues requiring large amounts of NADPH produced by this pathway are the tissues that synthesize fatty acids and steroids (e.g. mammary gland, adipose tissue, adrenal cortex and liver)

◦ Tissues less active in fatty acid synthesis such as skeletal muscle are virtually lacking the pentose phosphate pathway

Works in two phases: oxidative and non-oxidative

Pentose Phosphate Pathway: OxidativeGlucose-6-phosphate oxidized into ribulose-5-phosphate (and CO2)

During oxidation of glucose-6-phosphate, NADP+ is reduced to NADPH

The second step of the pathway coverts the ribulose 5-phosphate into other pentose-5- phosphates including ribose-5-phosphate used to synthesize nucleic acids

The third step converts three of the pentose-5-phosphates into two molecules of hexoses and one triose

In the fourth step, some of these sugars are converted into glucose-6-phosphate so the cycle can be repeated

◦ Three-carbon sugar is not recycled in the monogastric without FDPase(reverse glycolysis)

Pentose Phosphate Pathway Summary

Oxidative phase: 3Glucose-6-phosphate + 6NADP+ →2 Xylulose-5p + ribose-5P + 3CO2 + 6NADPH + 6H+

Rearrangements of the nonoxidative phase: 2 Xylulose-5P + ribose-5P →2 Frucose-6P + Glyceraldehyde-3P

Sum of these two phases: 3 Glucose-6-phosphate + 6NADP+ → 2 Frucose-6P + Glyceraldehyde-3P + 3CO2 + 6NADPH + 6H+

Pentose Phosphate Pathway Summary: Ruminants vs Monogastrics

Monogastrics:

3Glucose-6-phosphate + 6NADP+ → 2 Frucose-6P + Glyceraldehyde-3P + 3CO2 + 6NADPH + 6H+

Triose-P goes to TCA cycle

Ruminants:

12 glucose-6-P + 12 NADP -> 6 CO2 + 5 6-carbon units recycled + 12 NADPH

More glucose goes in but you recycle more carbons (due to FDPase) and still only use up 6 carbons released as CO2

Presence of FDPase allows recycling of triose-P (glyceraldehyde-3-P) so you produce 12 NADPH after the complete oxidation of one glucose equivalent (6 carbons)

Ruminants are more efficient at producing NADPH (for fatty acid synthesis) than monogastrics

Citric Acid CycleAKA Krebs cycle or tricarboxylic acid cycle (TCA cycle)

Purpose: TO PRODUCE ENERGY!

Starting point = pyruvate from Embden-Meyerhof pathway

Produces 30 ATP for each molecule of glucose oxidized

For each NADPH2 or NADH2 produced, 3 ATP are generated

Aerobic process

C3H4O3 + ½ O2 + 15 ADP + 15 Pi -> 3 CO2 + 2 H2O + 15 ATP

Citric Acid Cycle Videoshttps://www.youtube.com/watch?v=F6vQKrRjQcQ

https://www.youtube.com/watch?v=Fcu_8URp4Ac

Citrate lyase (8)

FDPase

Hey!! Don’t forget about me! I’m

used for energy production, too!!

Energy and Carbon Sources

Mammary Gland Primary Energy Source

Whole Body Primary Energy Source

C-SourceLactose

Blood GlucoseSource

C-SourceFatty Acids

Ruminant Glucose Acetate Glucose Propionate Acetate and butyrate

Monogastric Glucose Glucose Glucose Diet Glucose

What’s the Main Purpose?Glucose can go one of two ways:

Glycolysis (Embden Meyerhof Pathway): To produce pyruvate (2 pyruvates for every glucose) which will enter Citric Acid Cycle

Pentose Phosphate: To produce NADPH for fatty acid synthesis and ribose sugars for nucleic acid synthesis

Pyruvate goes to:

Citric Acid Cycle: To produce energy (NADH and FADH will eventually make ATP)

◦ Also produces oxaloacetate, citrate, isocitrate, malate, and 2-oxoglutarate (we will see these in the fatty acid lesson!)

Citrate lyase (8)

FDPase

Questions?