Metabolism

I just wanted to link what i learnt about metabolism over the past few weeks... so hear goes...

Fed state

Processes:

1) Glycogenesis

- high insulin levels

glucose -> glucose-6-phosphate -> glucose-1-phosphate -> glycogen

- I think the last stage of the pathway UTP energy is used to joint the phophate to join glucose onto a glycogen molecule...

2) Excess amino acids -

- i dunno what happens to these... are they excreted or converted into protein?

3) Fat synthesis

- Excess acetyl coA get converted into fatty acids, which are then stored as triacylglycerides

Fasting state

Processes:

1) Fat breakdown (lipolysis)

- lipases break ester bonds

- non-esterifed fatty acids undergo beta-oxidation (fatty acid -> acetyl coA)

- acetyl coA formed in respiration to release energy

- glycerol formed can be converted into glucose... dunno how?

2) Glycogen breakdown

Glycogen -> Glucose-1-Phosphate -> Glucose-6-Phosphate -> Glucose

- last stage of this reaction is catalysed by the enzyme glucose-6-phosphotase

- glucose-6-phosphotase is present in liver and so you can convert the glycogen reserves in the liver to glucose (in responce to high glucagon levels)

- this enzyme is not present in muscle and so you can only use glycogen in muscle for respiration starting with glucose-6-phopshate

(and not for increasing glucose levels... so you cannot utilise glycogen in muscle as an energy source for the rest of the body in the fasting state)

3) Gluconeogenesis

- The point of gluconeogenesis is to maintain blood sugar levels in the fasting state (so that glucose can be used as a source of energy for the brain and erythrocytes)

- Amino acids (from protein mainly in liver and muscle) cause an excess of oxaloacetate

(although gluconeogenesis also occurs by pyruvate -> oxaloacetate, but what is the point of this?!?)

- Can go through the reverse of glycolysis (well, partly) to form glucose but this can only occur in the liver cuz it has the necessary enzymes

4) Ketone synthesis:

- Excess fatty acids form ketones in the liver

- To provide a source of energy for muscle (that cannot obtain all its energy from fatty acids) thus saving glucose for brain / erythrocytes.

If you got this far, good for you... What I haven't got a clue about is...

fat synthesis, what happens to excess amino acids and what the pentose phosphate pathway is?

If I have missed or left out any important stages tell me... Cheers

Reply 1

corkskrew

Revenged

- glycerol formed can be converted into glucose... dunno how?

- Amino acids (from protein mainly in liver and muscle) cause an excess of oxaloacetate

(although gluconeogenesis also occurs by pyruvate -> oxaloacetate, but what is the point of this?!?)

If you got this far, good for you... What I haven't got a clue about is...

fat synthesis, what happens to excess amino acids and what the pentose phosphate pathway is?

Glycerol -> glucose
Glycerol's carbons can form an intermediate of glycolysis, DHAP (or Dehydroxyacetone Phosphate to give it it's full name)

Amino Acid/Pyruvate GNG
Amino acids can either:
-form intermediates of the TCA (Kreb's) cycle and form oxaloacetate
-form pyruvate (alanine mainly, but also serine)
*the pyruvate pathway of GNG is exactly the same as for lactate

Fat Synth
I think you're asking about triacylglycerol (truglyceride) synthesis, so I'll try and tackle this. Two main steps:

*Glycerol synth*
-Glucose enters liver
-Glycolysis occurs
-DHAP sythesised from on of the glycolysis intermediates
-Glycerol-3-phosphate made

*Fatty acid synth*
-Glucose enters liver
-Glycolysis all the way to pyruvate
-Pyruvate enters mitochondria, citrate formed
-Citrate cleaved to give oxaloacetate and Acetyl CoA
-A CoA
-Malonyl CoA synthesised from A CoA
-M CoA is used to add two carbons to the A CoA chain several times
-Key enzyme in the fatty acid synthase process is the A CoA carboxylase
-16 carbon chain is released from the process

*Then*
-Fatty acid chain is activated by making it a CoA derivative
-Glycerol-3-Phosphate then receives the FA CoA x3
(-As there's still a phosphate group on the 3rd glycerol carbon, the intermediate is Phosphatidic Acid. The group is released as inorganic phosphate)
-Triglyceride produced
-Apoproteins and other lipids added to create VLDLs which can then be released into the blood

Fed State Amino Acids
-Synthesis of endogenous proteins in the liver of other tissues (proteins undergo turnover so are always needed)
-Hepatic oxidation to glucose or ketone bodies (nitrogen then needs to be converted to urea)
-Synthesis of non-protein compounds that need amino acids, eg nonessential amino acids, hormones, neurotransmitters...

Not sure about pentose phosphate, not covered that yet!

Reply 2

Revenged

corkskrew

*the pyruvate pathway of GNG is exactly the same as for lactate

Cheers... v. v. helpful info (i probably go and learn some more details over the hoildays)

Anywayz, i was going to add that this is this gluconeogenesis 'oxygen debt' after exercise cuz i only just found out about it

1. Anaerobic respiration (during exercise)

Glucose -> Pyruvate -> Lactate

2. Aerobic gluconeogenesis

Lactate -> Pyruvate -> Glucose

The oxygen is only used for the first stage i think... but doesn't some of the pyruvate formed go in the Calvin cycle as well

Metabolism

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