Essential Metabolism Flashcards
What important factors influence enzyme activity in the body?
Covalent modification (eg phosphorylation)
Synthesis/degradation (protein availability)
Temperature
pH (ionic interactions influence protein shape)
Substrate availability [S]
Compartmentalisation (allows incompatible reactions to take place simultaneously within the cell)
Activators and deactivators (regulatory molecules)
Cofactors/coenzymes
What is the difference between cofactors and coenzymes?
Coenzymes are organic molecules that act as cofactors
How is energy stored in cells?
ATP
NAD(P)H
FADH2
Acetyl CoA
High-energy linkage
Phosphate
Electrons and hydrogens
Acetyl groups
Energy can also be stored as ion gradients and high energy phosphate bonds
What are redox reactions?
Reactions that involve electron transfer or can involve transfer of H enzymes are called dehydrogenases
What is the electron redox potential (aka E’)?
Measure of ability of one molecule to pass electrons to another.
More negative E’ indicates stronger reductant so more readily donates electron
What happens to energy when electrons are donated?
When donated from compound with more negative redox potential to one that is less negative/more positive energy is released
What causes abnormal metabolism?
Abnormal metabolism results from nutritional deficiency, enzyme deficiency, or abnormal secretion of hormones.
What are the main metabolic fuels?
Glucose and fats
Proteins can also be used
What is the pentose phosphate pathway?
A versatile pathway that does not make ATP but produces ribose 5-P for nucleotides
Supplies NADPH for reductive biosynthesis
Provides a pathway for metabolism of excess pentoses
Why do red blood cells need NADPH?
To reduce glutathione which is important for RBC integrity
What sections does the pentose phosphate pathway consist of?
Oxidative section which gets ribose-5P and NADPH
Nonoxidative section which manipulates the ribose-5P based on cell’s needs
What is considered when deciding the fate of the nonoxidative section of the Pentose Phosphate Pathway?
If the cell needs equal amounts of ribose-5P and NADPH oxidative PPP proceeds.
If the cell needs lots of NADPH but no ribose-5P then nonoxidative PPP converts excess ribose-5P to glucose-6P
What is the function of G6PDH?
It converts Glucose-6-phosphate to 6-phosphogluconate which is the first step of the oxidative phase of PPP.
What are some important functions of NADPH?
Reductive biosynthesis (form fatty acids, sterols, etc)
Glutathione production which allows cells to reduce oxidative damage by free radicals.
What happens to glucose in the absence of oxygen?
Fermentation occurs, pyruvate is produced via glycolysis and then the pyruvate is converted to lactate via lactate dehydrogenase.
What triggers the fermentation pathway?
Accumulation of NADH due to slowing down of electron transport chain due to lack of oxygen.
What is required for fermentation to take place?
Pyruvate and NADH
What happens to lactate that is produced?
It is acidic so it is exported via the cori cycle.
The lactate is exported into the blood and then processed by the liver and converted by LDH then gluconeognesis into glucose
Which enzyme phosphorylates glucose?
Hexokinase
Which tissues use glucose preferentially?
Muscles and the brain (only fuel for brain under normal circumstances)
What do ketone bodies get produced from?
From acetyl CoA
What are the key enzymes of glycolysis?
Hexokinase (converts glucose to glucose-6-phosphate)
Phosphofructokinase (provides energy for glycolysis to proceed; producing fructose 1,6-bisphosphate)
Pyruvate kinase (produces pyruvate in final step)
How is glycolysis regulated?
Citrate and ATP have a negative feedback action on phosphofructokinase and on pyruvate kinase.
Pyruvate kinase increases with more increase in fructose-1,6-bisphosphate to counteract the negative feedback of increased ATP and citrate.
What feedback mechanisms act on phosphofructokinase-1 in glycolysis?
ADP and AMP increase rate
ATP and citrate decrease rate (inhibitory)
What feedback mechanisms act on hexokinase?
Pi’s increase activity
G6P inhibit hexokinase-1
What feedback mechanisms act on pyruvate kinase?
ATP inhibits pyruvate kinase
Fructose-1,6-bisP increases its activity
How much glucose is needed by the brain?
120g/day
Where does most gluconeogenesis occur?
> 90% of gluconeogenesis occurs in the liver and the kidneys
How does gluconeogenesis occur?
It is the reverse of glycolysis except 3 enzymes (pyruvate kinase, fructose-1,6-bisphosphate, and hexokinase) are not used and these steps are bypassed by 3 other enzymes.
How do amino acids get converted into glucose?
They are converted to oxaloacetate and then undergo gluconeogenesis
How do fatty acids get converted into glucose?
Glycerol is converted into dihydroxyacetone phosphate which undergoes gluconeogenesis
Can acetyl-CoA be converted into glucose?
No, they can only be converted to ketone bodies if not in the TCA cycle
How is the reaction of pyruvate kinase reversed in gluconeogenesis?
2 reactions:
Pyruvate -> oxaloacetate via pyruvate carboxylase
Oxaloacetate -> Phosphoenolpyruvate via phosphoenol pyruvate carboxylase
What other substrate is required for Pyruvate -> oxaloacetate reaction?
ATP
What other substrate is required for Oxaloacetate -> Phosphoenolpyruvate reaction?
GTP
What enzyme catalyses the reverse reaction to phosphofructose kinase in gluconeogenesis?
Fructose-1,6-bisphosphatase
What enzyme catalyses the reverse reaction to hexokinase in gluconeogenesis?
Glucose-6-phosphatase
What is the overall reaction for gluconeogenesis?
2pyruvate + 4ATP + 2GTP + 2NADH + 6H2O -> glucose + 4ADP + 2GDP +6Pi +2NAD+ + 2H+
Where does glycolysis occur?
In the cytosol
Where does gluconeogenesis occur within the cell?
Pyruvate carboxylase - mitochondria
Phosphoenolpyruvate carboxykinase (GTP) - cytoplasm
Fructose-1,6-bisphosphatase - cytoplasm
Glucose 6-phosphatase - ER
Where is glucose-6-phosphatase located? why?
Glucose-6-phosphatase is only located in tissues responsible for maintaining blood glucose (liver and kidneys)
This is to ensure that the liver and kidneys store the glucose and don’t use it up.
What kind of control is G6phosphatase subject to?
Substrate level (i.e more substrate = faster reaction)
How many proteins is the glucose 6-phosphatase complex composed of?
5 proteins
Stabilising protein necessary for function
T1, 2, and 3 act as transporters in the ER
How are gluconeogenesis and glycolysis regulated relative to each other?
They are reciprocally regulated. Molecules that inhibit gluconeogenesis often activate glycolysis and vice versa
What molecules are important for regulation of gluconeogenesis?
Fructose 2,6-bisphosphate
AMP
ATP and citrate
What is fructose 2,6-bisphosphate?
A potent allosteric regulatory molecule which activates phosphofructokinase and inhibits fructose-1,6-bisphosphatase thus activating glycolysis and inhibiting gluconeogenesis
What does a high concentration of AMP indicate?
Cell’s energy charge is low and signals a need for ATP
What does high ATP and citrate indicate about a cell?
Cell’s energy charge is high and intermediates are abundant
What substrates can enter gluconeogenesis?
Pyruvate and molecules that can be converted into pyruvate
Lactate
TCA intermediates
Glycerol
Amino acids (most)
Galactose and fructose (all other monosaccharides can enter through these)
What molecules can not be substrates for gluconeogenesis?
Acetyl CoA
Fatty acids
Lysine and Leucine
What happens to the amino groups of amino acids after they are converted into substrates of glycolysis/gluconeogenesis?
They are converted into alpha ketoglutarate
Lactate and alanine are glucogenic. What does glucogenic mean?
Can be metabolised into glucose
What is the reaction that takes place between pyruvate and glutamate?
pyruvate + glutamate
alanine + α-ketoglutarate
Are amino acids glucogenic or ketogenic?
They can be glucogenic, ketogenic, or both
Which amino acids are glucogenic?
All but lysine and leucine are partly glucogenic
Which amino acids are ketogenic and glucogenic?
Aromatic AAs
Which amino acids are ketogenic but not glucogenic?
Leucine and lysine
What happens to fatty acids in metabolism?
They are converted to acetyl CoA which means they can’t be converted to glucose
What enzyme produces glycogen?
Glycogen synthase
How is glycogen formed from glucose?
UDP-glucose is formed containing uracil which helps extend the growing chain of glucose subunits
Where is pyruvate broken down into acetyl CoA?
in the matrix of the mitochondria
Which molecules can be converted into acetyl-CoA?
Glycogen
Fatty acids
Amino acids
How is pyruvate oxidised into Acetyl-CoA?
Via a multi-enzyme complex called pyruvate dehydrogenase
What regulation happens in high energy to pyruvate dehydrogenase?
ATP
Acetly CoA (molecule inhibits its own production)
NADH
What regulation happens in low energy to pyruvate dehydrogenase?
Pyruvate increases PDH activity
ADP also increases PDH activity
How is pyruvate dehydrogenase regulated covalently?
PDH kinase and phosphatase
Where does the TCA cycle take place and what happens in cells that don’t undergo the TCA cycle much?
In the mitochondria. Cells with little or no mitochondria rely on glycolysis primarily
What are the products of the TCA cycle?
One turn of the cycle produces 3 NADH, 1 GTP, and 1 FADH2
Releases 2 CO2
Which tissue is the most important tissue for metabolism?
Liver is the only tissue in which all metabolic processes occur significantly
What is fumarase deficiency?
Fumarase deficiency is an autosomal recessive disorder causing severe neurological impairment, encephalopathy and dystonia
What abnormalities can affect the TCA cycle?
Very few genetic (incompatible with life)
TCA is aerobic so anoxia or hypoxia lead to total or partial inhibition of the cycle
4 B vitamins have precise roles in TCA cycle. Riboflavin (FAD), Niacin (NAD), Thiamine (alpha-KG) and pantothenic acid (CoA)
How many complexes are in the electron transport chain?
5, 4 electron transporters and ATP synthase
How do electrons get carried by complexes?
They have haem groups
What does the proton gradient do?
Voltage gradient drives ADP-ATP exchange
pH gradient drives pyruvate entry into mitochondria and phosphate import
How is oxidative phosphorylation regulated?
Cellular energy demands dictate ETC.
Intracellular [ADP]
& ratio of [ATP] : ([ADP][Pi]). When the cell needs more
ATP, the ETC is activated by high [ADP] & [Pi].
[ATP] & [ADP] set the rate of e- transfer through
ETC via a series of coordinated controls on respiration,
including glycolysis & TCA cycle.
What do carbon monoxide and cyanide do?
Binds to Fe of cytochrome oxidase (Complex IV) preventing electron transport and thus preventing ATP production via oxidative phosphorylation
What does antimycin A do?
blocks electron transport to cytochrome c1
What does rotenone do?
Blocks NADH from binding to complex 1
What are the mechanisms of disease in metabolism?
Accumulation of toxin
Energy deficiency
Deficient production of essential metabolite/structural component
How are inherited metabolic disorders detected?
Heterozygote carriers are often found during screening of family members of an affected individual.
Prenatal diagnosis (eg cystic fibrosis)
Neonatal screening (Eg PKU)
Neonate developing symptoms in first few weeks
Adult onset (huntington’s disease)
What kind of genetic defects are most commonly seen in inherited metabolic disorders?
All feature a genetic defect (often a result of single base substitution or deletion), reduced synthesis or absence of particular protein or D in 1° structure.
More than 4000 disorders involving single genes.
Most inherited disorders are autosomal recessive,
heterozygotes usually normal.
Many diseases have a defective enzyme that causes the disorder
True or False:
Most Inborn Errors of Metabolism (IEM) diseases are recessive and so a negative family history is not reassuring
True
How are IEM disorders investigated?
Consanguinity, ethnicity
Neonatal deaths, foetal deaths
Maternal family history:
Males - X linked disorders
All - mitochondrial DNA is maternally inherited
True or False:
A positive family history may be helpful.
True
What kind of disorders are IEMs most commonly?
Enzyme deficiencies
What are possible consequences of enzyme deficiency?
Decrease in product
Accumulation of metabolite
Increased formation of other metabolites
What are the symptoms of inherited disorders of metabolism?
Non-specific symptoms include:
Vomiting, lethargy
Failure to thrive, feeding difficulties
Respiratory distress
Hypotonia, seizures
Hypothermia
Other “telling”signs include:
Ocular findings
Hepatosplenomegaly
Cardiomyopathy
Abnormal odour,
Unusual skin &/or hair
What enzyme is absent in classical galactosaemia?
UDP - glucose:galactose 1-P uridyltransferase
How is galactose metabolised?
Milk -> Lactose -> Glucose + Galactose -> Galactose - 1-P —(UDP-glucose:Galactose-1-P uridyltransferase)—> Glucose-1-P -> Glucose-6-P -> Pyruvate
How is classical galactosaemia treated?
Galactose free diet and opthalmology and developmental follow up
What are the symptoms of classical galactosaemia?
Mental retardation
Jaundice
Hepatomegaly
Cirrhosis
Infantile cataracts
Hepatic failure and death
What are the other causes of galactosaemia?
Galactokinase deficiency (galactitol is produced which causes infantile cataracts
Galactonate production
