GI Biochem Flashcards
Glycogenesis
Synthesis of glycogen from glucose
Main storage form
Liver glycogen
Muscle glycogen
Liver glycogen /
• Broken down between meals and released to maintain blood glucose levels for RBCs and brain
Muscle Glycogen /
- Not available for maintenance of blood glucose levels
- Provides energy via glycolysis and the TCA during exercise
The liver keeps blood/sugar levels…
Constant between meal times
What process happens at night when liver glycogen stores have been depleted?
Gluconeogenesis
What is at the centre of glycogen? (protein)
Glycogenin
Catalytic activity
Can add up to 4 glucose molecules to itself
Glucose can only be added to…
an existing glycogen chain
What is needed for glucose to attach?
A glycogen “primer”
This primer is covalently attached to glycogenin
Glucose –> G-6-P catalysed by?
Hexokinase
and ATP
G-6-P –> G-1-P
By phosphoglucomutase
G-1-P –> UDP glucose
UDP glucose is the activated form of glucose
by pyrophosphorylase
UDP glucose acts as a substrate for
Glycogen synthase
to gie GLYCOGEN + UDP
How many ATP are used to attach one glucose to glycogen?
1 ATP molecule
Glycogen synthase is the ____ limiting enzyme of glycogenesis
Rate limiting
Branching enzyme (introduces an alpha 1,6 glycosidic branch into glycogen. Which enzyme
Transglycosylase
Glycogenolysis
Breakdown of glycogen
Catalysed by glycogen phosphorylase
One glucose molecule is cleaved off the ends at a time
G-1-P –> G-6-P by phosphglucomutase
Fates of glycogenolysis?
G-6-P —– g-6-p-atase —> Glucose —> Blood (via GLUT2)
Glycolysis to muscles
INSULIN
Stimulates synthesis of glycogen
Stimulates synthase (storage)
Inhibits phosphorylase
inhibits gluconeogenesis
stimulates glycolysis
GLUCAGON
Signals lack of glucose in blood stream
Stimulates glycogen phosphorylase (breakdown)
Inhibits glycogen synthase
stimulates gluconeogenesis
Adrenaline & cortisol
Both stimulate glycogen phosphorylase (breakdown)
Gluconeogenesis
what is it?
when does it happen?
Synthesis of glucose within the body from NON-CARBOHYDRATE precursors
Prolonged starvation (when glycogen stores are depleted)
Gluconeogenesis - precursor molecules
Lactate - anaerobi cconditions
Amino acids - derived from muscle protein by proteolysis
Glycerol - derived from triglycerides by lipolysis in adipose tissue
Is gluconeogenesis energy consuming?
Very much
6 ATP
Is gluconeogenesis the reverse of glycolysis?
NOOOOOOOOOOOOO
How many enzymes does gluconeogenesis require?
4
Proceeds via synthesis of OXALOACETATE in mitochondria
Lactic acid (cori) cycle - lactate (precursor)
Precursor for gluconeogenesis (anaerobic)
Blood transports lactate to liver
Liver converts lactate to glucose
Amino acids (precursor molecule)
2 classes/
- Ketogenic - can’t be used for making glucose
- Glucogenic - CAN be used for making new glucose
Lipid catabolism
Increased fat intake
↑ No of adipocytes
more fat in adipocytes
–> Disease
Lipids
Simple
Compound
Steroid
> Simple lipids (fatty acids, triglycerides, waxes
> Compound lipids (phosphor, glycol, lipo-protein)
> Steroids (cholesterol, steroid hormones)
Triglycerides
Main energy storage form
Hydrophobic
Glycerol + 3 fatty acids
Fatty acids can be…
Saturated
Unsaturated
Polyunsaturated (Several double bonds)
Usually in cis config. (hydrogen on same side)
Unsaturated fats are essential/non-essential
ESSENTIAL
What is the main product of fat digestion?
Glycerol
absorbed into mucosal cells of intestine
What happens to short and medium length fatty acids?
They enter portal blood
Longer chains and monoglycerides are resynthesised –> triglycerides
What happens to triglycerides?
Coateed with protein, phospholipid and cholesterol –> CHYLOMICRONS
enter lymph
then blood stream
at muscle and adipose tissue chylomicrons are cleared by lipoprotein lipases
Fatty acid oxidation
Occurs in cytoplasm
Requires 2 ATP
Fatty acid + CoA —> Acyl -CoA (not acetyl CoA)
Further oxidation in mitochondra
What shuttle moves fatty acids from acyl-CoA to the mitochondria
Carnitine shuttle
acyl carnitine into mitochondrion
Acyl breaks off and carnitine is carried out of cell.
Acyl binds with CoA in mitochondrion to from acyl CoA again.
Beta-oxidation
Acyl CoA is substrate
Massive energy gain
Ketone bodies
Formed in liver mitochondria
Diffuse into bloodstream and peripheral tissues
Molecules of energy metabolism for heart muscle and renal cortex
Ketosis
Starvation or diabetes
acetyl CoA converted to ketone bodies because oxaloacetate has been used in gluconeogenesis
Leads to acidosis
Impairs CNS –> coma
Smell of acetone on breath
DEATH
Lipogenesis
Liver
Excess energy intake
reductive process
How are free fatty acids transported in the blood?
Albumin
What enzyme catalyses synthesis of long chain fatty acids?
Fatty acid synthase
From malonyl CoA, acetyl CoA and NADPH (electron donor)
Palmitic acid is the
longest faty acid produced by FAS
What happens to amino acids that are not used as building blocks?
Degraded (not stored) in LIVER
What are the toxic bi products of amino acid breakdown?
NH3 and NH4+
Excreted in urea
Urea synthesis
3 steps
- Transamination/
- amino group of an amino acid is transferred onto a keto acid
- forms a keto acid and a new amino acid
- De-amination/
- removal of amino group from glutamic acid in liver
- resulting in formation of alpha-keto gluterate
- Urea cycle
- free ammonium ion is converted to urea
- second nitrogen is derived from aspartic acid
(Urea N2H4CO)
What are the major carriers of nitrogen in the blood?
Alanine and glutamine
Carbon skeletons
Removal of amino group leaves a carbon skeleton.
Converted into major metabolic intermediates
Can be used for gluconeogenesis or oxidised in TCA cycle
Alcaptonuria
Degradation of phenylalanine and tyrosine is blocked
Maple syrup wine disease
Degradation of valine, isoleucine and leucine is BLOCKED
urine smells like maple syrup
mental and physical retardation
appropriate diet
Phenylketonuria
Phenylalanine accumulates in all body fluids
mental retardation
therapy - low phenylalanine diet
