changes to metabolism Flashcards
describe the fuel storage capacity for the body for a 70kg man
For a 70kg man:
Glycogen= 0.2kg= 800kcal
Triacylglycerol=15kg=135,000 kcal
Protein=6kg=24,000 kcal
how many amino acids are there
20
essential and non essential
what are essential amino acids
cannot be produced by the body
Arginine
Histidine
Methionine
Threonine
Valine
Leucine
Lysine
Isoleucine
Phenylalanine
Tryptophan
what are non essential amino acids
Can be produced by the body
Alanine
Asparagine
Aspartate
Cysteine
Glutamate
Glutamine
Glycine
Proline
Serine
Tyrosine
what 3 categories can essential amino acids be divided into
- glucogenic:
Arginine
Histidine
Methionine
Threonine
Valine
- ketogenic
Leucine
Lysine
- glucogenic + ketogenic
Isoleucine
Phenylalanine
Tryptophan
what 2 groups can non essential amino acids be divided into
- glucogenic
Alanine
Asparagine
Aspartate
Cysteine
Glutamate
Glutamine
Glycine
Proline
Serine
- glucogenic + ketogenic
Tyrosine
explain alanines conversion to pyruvate
Alanine loses its amino group by transamination to form pyruvate catalysed by alanine aminotransferase
explain asparagines conversion to oxaloacetate
Asparagine is hydrolysed by asparaginase, liberating ammonia and aspartate.
Aspartate loses its amino group by transamination via the enzyme aspartate aminotransferase to form oxaloacetate.
explain glutamines conversion to alpha-ketoglutarate
Glutamine is converted to glutamate and ammonia by the enzyme glutaminase (2).
Glutamate is converted to -ketoglutarate by oxidative deamination by glutamate dehydrogenase (1).
explain tyrosines conversion to fumarate
A multi-step reaction
Transamination
- Tyrosine –>Hydroxy-phenylpyruvate
- ⍺-ketoglutarate—> Glutamate
Dioxygenation:
- Hydroxy-phenylpyruvate—> Homogentisate
- O2 + Ascorbate (Vit. C)–> CO2 + H2O
Dioxygenation:
- Homogentisate–> 4-Maleylacoacetate
Isomerisation:
- 4-Maleylacoacetate–> 4-Furmarlacoacetate
Hydrolysis:
4-Furmarlacoacetate–> Fumarate and Acetoacetate
explain the causes of starvation
Inability to obtain food
Desire to lose weight
Clinical Situations:
- Trauma (shock)
- Burns
- Injury to face
- Tumour
explain what happens st metabolic level during starvation
Blood levels of amino acids, glucose, and triacylglycerols fall
Blood insulin levels are very low, glucagon levels are very high
Both factors trigger a period of catabolism, characterised by the degradation of:
- Glycogen –> Glucose
- Triacylglycerol –> Fatty Acids and Glycerol
- Protein –> Amino Acids
This results in an interchange of substrates between:
Liver
Adipose tissue
Skeletal Muscle
Brain
what determines the fates of the substrates interchanged during metabolic level of starvation
The fate of these substrates is determined by 2 crucial factors:
- The essential need to conserve glucose for those cells &tissues that really need it, such as red blood cells and brain.
- The need to mobilise fatty acids from adipose tissue andketone bodies from liver to supply energy to all othertissues, and for these other tissues to adapt to these non-glucose substrates.
describe the enzymatic changes in starvation
In all situations the flow of intermediates through biochemical pathways is controlled by 4 mechanisms:
- Availability of substrates
- Allosteric activation/inhibition of enzymes
- Covalent modification of enzymes
- Reciprocal Induction or repression of enzyme synthesis
explain the carbohydrate metabolism that occurs to the liver during starvation
Initially glycogen breakdown, then gluconeogenesis to meet the following objectives:
- Maintain blood glucose levels
- Sustain energy provision for the brain and other glucose requiring tissues (e.g. red blood cells).
explain increased glycogen degridation in the liver during starvation
After a meal glucose from food is the major source of blood sugar.
A few hours after a meal blood glucose levels start to decline.
Insulin levels drop, glucagon levels increase which stimulates (by cAMP cascade) glycogen breakdown in liver.
Liver glycogen stores will last for 10-18 hours of fasting.
explain liver, increased gluconeogenesis that occurs during starvation
Unique ability of liver to synthesise glucose is vital during starvation and becomes increasingly active as glycogen stores are depleted.
Carbon skeletons are derived from:
- Glycerol
- Lactate
- Amino acids
explain lipid metabolism in liver starvation
Increased fatty acid oxidation
- Oxidation of fatty acids derived from adipose tissue is the major source of energy for liver during starvation (this spares glucose)
- Increased synthesis of ketone bodies
Ability of the liver to synthesise and release ketone bodies (acetyl units) is unique.
- Synthesis is favoured when [acetyl CoA] produced via fatty acid oxidation exceeds the maximal rate of the citrate cycle.
- KBs are vital in starvation because they can be used by other tissues and cells provided they have mitochondria.
- Once the level of KBs in the blood is high enough, up to ⅔ of the brain can use them as fuel.
- Utilisation of KBs reduces the need for amino acid precursors for gluconeogenesis and this decreases protein breakdown.
explain carbohydrate metabolism of adipose tissue in starvation
In starvation, insulin levels decrease and hence glucose is not used for fatty acid synthesis (no signal).
explain lipid metabolism of adipose tissue in starvation
Increased degradation of TAGs
- decrease insulin & increase glucagon (via cAMP cascade) causes phosphorylation and activation of hormone-sensitive lipase.
Increased release of fatty acids
- Fatty acids obtained from hydrolysis of stored adipose TAGs are released into the bloodstream.
- Bound to albumin, they are transported to lots of tissues for use as a fuel.
- The glycerol that is produced as the result of complete TAG hydrolysis is used for gluconeogenesis by the liver.
explain carbohydrate metabolism in skeletal muscle in starvation
Glucose requirements of muscle are decreased during starvation due to very low insulin levels
explain lipid metabolism in skeletal muscle during starvation
During first couple of weeks of starvation, muscle uses fatty acids from adipose tissue and ketone bodies from the liver as fuels.
Beyond this time period, muscle utilises only fatty acids as a fuel.
- This spares ketone bodies for other tissues
- Helps promote greater increase [KB] in the blood so that some parts of the brain can utilise them
explain protein metabolism in skeletal muscle during starvation
During first few weeks of starvation there is a lot of muscle wastage due to protein breakdown.
Resulting amino acids are used for gluconeogenesis.
Because the brain can utilise KBs when the [KB] in the blood is high enough, there is less of a requirement for the liver to perform gluconeogenesis.
The knock-on effect of this is that further muscle protein degradation is paused as blood [glycerol] and [lactate] are sufficient to meet the gluconeogenic needs of the liver.
explain the brain in starvation
First few weeks, brain 100% dependent on glucose.
Later, as [KB] rises, can adapt to using ketone bodies for ⅔ of its caloric requirements – reducing the need for glucose consumption.
what is metabolic disease
Inborn Errors of Metabolism
Inborn Error = An inherited genetic disorder which is either:
- Autosomal recessive
- X-linked
what are metabolic diseases majorly due to
to defects in single genes that code for enzymes that facilitate conversion of various substrates into other products
The term inborn error of metabolism was coined by who
garrod in 1908
what did garrods work prefigure
the ‘one gene one enzyme hypothesis’ based on his studies on the nature and inheritance of alkaptonuria.
in most metabolic disorders problems arise due to what
arise due to accumulation of substances which are toxic or interfere with normal function, or to the effects of a reduced/nonexistant ability to synthesize essential compounds.
explain garrods hypothesis
He developed an increasing interest in chemical pathology, and investigated urine chemistry as a reflection of systemic metabolism and disease.
This research, combined with the new understanding of Mendelian inheritance, evolved from an investigation of a few families with an obscure and not very dangerous disease (alkaptonuria) to the realization that a whole territory of mysterious diseases might be understood as inherited disorders of metabolism.
Over the next decade he developed an understanding of the possible nature of inherited diseases of metabolism.
He formulated the “one gene, one enzyme” hypothesis and described the nature of recessive inheritance of most enzyme defects. In 1908, the core of this work was presented as the Croonian lectures to the Royal College of Physicians, entitled Inborn Errors of Metabolism and published the following year.
Garrod expanded his metabolic studies to cover cystinuria, pentosuria, and albinism. In 1923 he summarized these studies in an expanded edition of his best known work,
what is the number of overall incidence of inherited metabolic disease
is about 40 in 100 000 live births (1 in 2500)
explain inherited metabolic diseases
Metabolic diseases are individually rare, but as a group are not uncommon
There presentations in the neonate are often non-specific at the outset
Many are treatable
Most difficult step in diagnosis is considering the possibility
what are the major categories of inherited metabolic diseases
Disorders of amino acid metabolism
Disorders of carbohydrate metabolism
Lysosomal storage diseases
- Lysosome = Organelles containing enzymes for breaking down biological polymers
Disorders of peroxisomal function
- Peroxisomes = Organelles specialised in oxidation/reduction reactions using O2
Disorders of mitochondrial function
Miscellaneous (not exhaustive):
- Pelizaeus-Merzbacher disease (impaired myelin synthesis)
- Hallervorden-Spatz disease (pantothenate kinase deficiency – neurodegenerative)
- Canavan & Alexander’s diseases (myelin phospholipid impairment)
- Van der Knaap leukodystrophy (impaired myelination of brain)
explain how inherited metabolic diseases present in the neonate
Acute life-threatening illness, no obvious diagnosis
Encephalopathy (large brain)
- Lethargy, irritability, coma
Vomiting
Respiratory distress
Seizures & hypertonia
Hepatomegaly (enlarged liver)
Hepatic dysfunction / jaundice
Odour, Dysmorphism, Failure to Thrive, Hiccoughs
Ambiguous genitalia
explain what inherited metabolic disease looks like in the infant
Failure To Thrive
Mental retardation
Severe irritability
Impulsivity
Aggressiveness
Hyperactivity
Progressive psychomotor retardation
how is metabolic disorder suspected
In any un-well, full-term infant who has no antecedent maternal fever or PROM (premature rupture of membranes)
Sick enough to warrant a blood culture or lumbar puncture
what are the lab tests done in suspicion of a metabolic disorder
Glucose, electrolytes, blood gases, ketones, BUN (blood urea nitrogen), creatinine
Lactate, ammonia, bilirubin, LFT
Amino acids, organic acids, reducing substances
explain the index of family history being an suspicion for metabolic disorders
Most IMDs are recessive
Consanguinity, ethnicity, inbreeding
Previous neonatal loss, miscarriage
Maternal family history:
- Males – X-linked disorders
- All – mitochondrial DNA is maternally inherited
how can history be an index of suspicion for metabolic disorders
Can the symptoms be explained?
Timing of onset of symptoms
- After feeds were started?
Response to therapies
explain lab investigation of metabolic disorders
Clinical Biochemistry is highly automated
- Anion gap metabolic acidosis
- Normal anion gap metabolic acidosis
- Respiratory alkalosis
- Low BUN : Creatinine
- Hypoglycaemia
Esp. with hepatomegaly
Non-ketotic
name the biochemical classifications of inherited metabolic disorders
Small Molecule Disease
- Carbohydrate
- Protein
- Lipid
- Nucleic Acids
Organelle
- Lysosomes
- Mitochondria
- Peroxisomes
- Cytoplasm
what are the 3 types of inherited metabolic disorders
Type 1 – Silent Disorders
Type 2 – Acute Metabolic Crises
Type 3 – Neurological Deterioration
explain the inherited metabolic disorder, type 1 - silet disorder
Do not manifest in life-threatening crisis
Untreated could lead to brain damage developmental disabilities
Examples include IMDs associated with the metabolism of the aromatic acids
explain Phenylketonuria (PKU)
Inability of the body to use the essential amino acid phenylalanine
Characterised by an elevated blood phenylalanine levels
3 different phenotypes:
- Classic PKU - blood Phenylalanine levels of > 1200 µmol/l
- Variant PKU - blood Phenylalanine levels of 600 – 1200 µmol/l
- Mild Hyperphenylalanemia (HPA) - blood Phenylalanine levels of < 600 µmol/l
explain Phenylalanine Hydroxylase (PAH) Mixed function oxidase
Catalyses the hydroxylation of phenylalanine, resulting in the formation of tyrosine
what is the number of classic PKU incidence
1/10,000 – 20,000 Caucasian or Asian births
how is classic pku inherited
Inherited as an autosomal recessive trait of a defective PAH gene
- More than 500 different mutations at the PAH gene locus have been identified
- Heterozygotes (1.5%) are normal
explain metabolite profiles in classic pku
Carriers of Classic PKU have a reduced level of PAH, reflected as an increased level of phenylalanine in the blood and the brain and a reduced level of Tyrosine
Untreated blood phenylalanine levels >1200 µmol/L is neurotoxic
Accumulating Phe is metabolised via alternate routes, leading to phenylpyruvate and phenyllactate
Disruption of amino acid transport (Tyr and Trp) across the blood brain barrier, reduction in catecholamine biosynthesis and accumulation of toxic metabolites.
explain mild pku
The cause is the same classical PKU, a mutation in the gene for Phenylalanine Hydroxylase however the result is not as severe
- Less functional enzyme
- Less of the functional enzyme
explain variant pku
The mutation is not in Phenylalanine Hydroxylase but in Dihydrobiopterin Reductase, the enzyme that reduces and recycles BH2 for the oxidation of Phenylalanine
explain diagnosis of Phenylketonuria
Early screening of neonates in most developed countries e.g. Guthrie test (bacterial inhibition test) most common
- Guthrie Test:
Neonatal Heel Prick test
Bacillus subtilis is grown on minimal culture media agar plates containing B-2-thienylalanine
This inhibits growth unless Phenylalanine is added
Neonatal blood is drawn onto a card, and a punch is made into that and placed onto the agar.
Should phenylalanine be present in a high concentration the bacteria will grow under the card
Useful for the diagnosis of a multitude of conditions.
Newer methods by tandem MS/MS to measure the concentration of Phe and the Phe/Tyr ratio
explain the Phenylketonuria Symptoms
A musty odour on the breath, skin, and urine caused by the increased [phenylalanine]
Neurological problems that may include seizures
Eczema
Fair skin and blue eyes due to the lack of melanin
Microcephaly
Hyperactivity
Delayed development and intellectual disability
- If PKU not detected within the first month of life, then the delayed development and mental retardation becomes irreversible
Behavioural, emotional, and social problems
Psychiatric disorders
explain treatment of pku through diet
Primarily by the restriction of Phe intake through natural protein restricted diets supplemented with phenylalanine free amino acid mixtures
British National Formulary outlines types of “Food for Special Diets” available within the NHS
- Such food preparations are regarded as drugs and include a range of protein liquid supplements and Tyrosine amino acid supplements
Ideally, gene therapy or enzyme replacement therapy should become available and is an area of active research
explain the Treatment for PKU – BH4 cofactor approach
Some PKU sufferers benefit from oral tetrahydrobiopterin (BH4) administration
- BH4 responders
Stable, synthetic BH4 formulation has been approved by Food and Drug Administration
Available in tablet form as Kuvan (Sapropterin Dihydrochloride)
- Dosing of 20mg/kg required for the ingestion of up to 14 tablets per day
- Led to some non-compliance in a recent study
Treatment lowers the concentration of blood Phe concentrations
what is alkaptonuria
Also known as ‘Black Urine Disease’
Autosomal recessive disease in which the enzyme Homogentisic acid oxidase is absent
Blood levels of homogentisic acid become extremely high and thus it is excreted in the urine
Urine samples darken with time as homogentisic acid oxidises to the black coloured alkapton, hence alkaptonuria
what are the consequences of alkaptonuria
Homogentisic acid accumulation in tissues leads to:
Cartilage damage and ochronosis leading to lower back pain in child sufferers.
- Need for hip and knee replacement in young sufferers
- Ochronosis – yellow discolouration of the tissue seen microscopically
Heart valve abnormalities (aortic stenosis)
Kidney and prostate stones
Darkening of the sclera of the eyes
Ear wax exposed to the air turns red or black
what are the treatments for Alkaptonuria
Large dosage of ascorbic acid (Vitamin C)
Dietary restriction of phenylalanine and tyrosine may be effective in children, but benefits in adults have not been demonstrated
The herbicide nitisinione inhibits p-hydroxyphyenylpyruvic acid oxidase and thus decreases the amount of homogentisic acid produced
- Currently undergoing trials in various methods of administration
- The main side effect is irritation of the cornea and is a concern that it will cause the symptoms of tyrosinemia because of the possible accumulation of tyrosine or other intermediates
explain IMD Type 2 – Acute Metabolic Crisis
Life threatening in infancy
Children are protected in utero
- Maternal circulation which provides the missing product or removes the toxic substance
Example: Urea Cycle Disorders
explain ammonia and the urea cycle
Ammonia is generated from a variety of sources in the body
- It is a waste product of the deamination of amino acids
- It is also produced in large quantities by gut bacteria
It is absorbed across the intestinal wall and found in high concentrations in the hepatic portal blood
It is produced by the metabolism of muscles and venous concentrations are higher than arterial
The Urea Cycle enables toxic ammonia molecules to be converted to the readily excreted and non-toxic urea
- It has other metabolic benefits - important source of arginine, which is used in a variety of metabolic reactions
what is urea
is the major disposal form of amino groups derived from amino acids and accounts for the majority of the nitrogen-containing components of urine
where is urea produced and transported and why
Urea is produced in the liver and is transported in the blood to the kidneys for urinary excretion
name the enzymes found in the mitochondria of the urea cycle
① Carbamyl phosphate synthase I
② Ornithine transcarbamylase
name the enzymes found in the cytosol of the urea cycle
③ Arginosuccinate synthase
④ Arginosuccinate lyase
⑤ Arginase
what happens if the urea cycle goes wrong
Defects of enzymes involved in the urea cycle lead to hyperammonaemia and an arginine deficiency, except in the case of arginase deficiency
what is ammonia which is produced by the urea cycle going wrong
Ammonia is neurotoxic and damages the central nervous system, causing a variety of symptoms from drowsiness to death
what enzymes are not present if there is a mitochondrial enzyme deficiency
① Carbamyl phosphate synthase I
② Ornithine transcarbamylase
explain mitochondrial enzyme deficiencies
Represent the most severe of the Urea Cycle Disorders
Appear unaffected at birth
As hyperammonaemia progresses vomiting will begin after a few days, followed by respiratory distress, lethargy, and may slip into a coma
Untreated = Death
Treated = Regular Relapse, severe developmental disabilities
explain Ornithine Transcarbamylase Deficiency
The most common urea cycle disease
It is X-linked, thus there is a variable phenotype in female heterozygotes depending on pattern of random X chromosome inactivation
Males are usually more severely affected
Characterised by orotic aciduria and hyperammonaemia
Amino acid abnormalities are mainly non-specific, i.e., increased glutamine and alanine, and decreased ornithine, arginine, and citrulline
explain the cytosolic enzyme deficiency Citrullinaemia
no enzyme ③ Arginosuccinate synthase
Characterised by:
- Elevated Citrulline in plasma and urine
- Orotic acid in the urine
- Hyperammonaemia
Treatment is slightly easier because sufferers are able to incorporate some waste nitrogen into urea cycle intermediates
explain the cytosolic enzyme deficiency Arginosuccinic aciduria
no enzyme ④ Arginosuccinate lyase
Characterised by elevations of argininosuccinate in plasma and urine
Renal excretion provides the body with route to excrete nitrogen, so hyperammonaemia is often mild and may be absent
explain the cytosolic enzyme deficiency Hyperarginaemia
no enzyme ⑤ Arginase
It is characterised be elevations of arginine in plasma and urine, and orotic aciduria
Hyperammonaemia is variable and may only be mild or intermittent
However, the clinical picture is usually severe
- Patients may present with neonatal seizures and frequently suffer progressive neurological symptoms as they grow. This includes spastic diplegia, a subtype of cerebral palsy
name the treatment of urea cycle diseases
Alternative pathway stimulation
Haemodialysis, in cases of acute, extreme hyperammonaemia
Stimulation of Carbamyl Phosphate Synthase by a synthetic cofactor
A low protein diet is a very common strategy to control chronic hyperammonaemia
Arginine supplementation in relevant disorders
Liver Transplantation
explain the Urea Cycle Disease Treatment - Alternative pathway stimulation
Oral drugs that cause an increase in the excretion of glycine thereby depleting ammonia by stimulating the replacement synthesis of glycine using ammonia as a substrate
Most commonly used drugs:
- Benzoate
- Phenylbutyrate
- Phenylacetate
explain IMD Type 3 – Progressive Neurological Deterioration
Early childhood
- Progressive loss of motor and cognitive skills
Infant
- Non-responsive state
Infant/Adolescence
- Death
explain IMD type 3s Tay sachs disease
Lack of an enzyme called Hexosaminidase A
- Gangliosides in neural tissue cannot be degraded
what happens in tay sachs disease from the normal birth to the death
normal birth
lack of interaction/startle reflex
weak floppy, muscle blindness
seizures
shallow breathing
paralysis
death
