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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?