Principles of Biochemistry 21 |Brain Metabolism| Class Notes |HarvardX

Hormones regulate metabolism

A similar hormone: Adrenaline
Adrenaline works similarly to glucagon. But instead of the liver, the muscle is the key target of adrenaline.

Growth hormone could also affect blood levels. It is derived from the gradual consumption of fatty acids and not by gobbling glucose.

Thyroid hormones (thyroxine) could both regulated anabolic and catabolic pathways.
Overdose:

• Metabolic overdrive
• Thyroid hypertrophy
• Weakness, sweating, weight loss, irregular heartbeat.
  Limited dose:
• Incidence increases with age
• Slow metabolism
• Weight gain, fatigue, cold sensitivity
  Function:
• Activates noradrenaline release
• Noradrenaline stimulates thermogenin in brown fat
• Promotes uncoupling of ETC
  • Heat production
• In other tissues: promotes simultaneous synthesis and degradation of molecules.
  • Heat production

Conclusion:

• Tissue specificity of action
• Permits coordination between tissues
• Critical for maintaining homeostasis

Glucocorticoids:
They promote survival in the time of stress and scarcity by slowing the rate of nucleic acid synthesis and protein synthesis.

	Insulin	Glucagon	Adrenaline	Growth Hormone	Thyroid Hormone	Cortisol
Origin	$\beta$-cells	$\alpha$-cell	Adrenal medulla	Pituitary gland	Thyroid gland	Adrenal cortex
Stimulus	$\uparrow$ [Glucose]blood	$\downarrow$[Glucose]blood	Fear, stress, exercise	Age-dependent	Circadian	Prolonged stress
Liver	- Glycogen synthesis - Glycolysis - Fatty acid synthesis	- Glycogenolysis - Gluconeogenesis - Glucose Release	Glucagon	- Ketogenesis - Gluconeogenesis - Glycogen synth	- Glycogenolysis - Gluconeogenesis	- Gluconeogenesis - Glycogen synthesis - Fatty acid oxidation
Muslce	- Glucose uptake - Glycogen synth - Amino acid synth	Reduce glucose uptake	- Glycogenolysis - Glycolysis	- AA uptake - AA synth	- Proteolysis - Amino acid synth	- Reduce glucose uptake - Proteolysis
Adipose	- Glucose uptake - Fatty acid synth	- Reduce glucose uptake - Release fatty acid	Release fatty acid	- Lipolysis - Reduced glucose uptake	- Lipolysis - Fatty acid synth	- Reduce glucose uptake - Lipolysis
Blood Glucose	Decrease	Increase	Glucagon	stable	stable	stable
Free Fatty Acid	Decrease	Increase	Glucagon	Increase	stable	stable

Blood-brain barrier

Capillaries

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Capillaries are the smallest blood vessels formed by a ring of endothelial cells. The structure of brain capillaries is very different. Some large molecules pass through fluid channels called fenestra directly from the blood to the interstitial fluid. But capillaries in the brain don’t have fenestra.

• Gases and water:
  In the brain, H₂O, CO₂, and O₂ are diffuse freely across the endothelial.

• Fuel molecules

  • Glucose
    Glucose is transported by facilitated diffusion witch carried by GLUT1. It was 50,000 faster than sample diffusion. It was also found in cardiac muscle and red blood cells.
  • monocarboxylic acid
    lactate, pyruvate, and ketone bodies.
    facilitated diffusion: MCT1
  • essential fatty acid
    e.g., $\Omega$ 3 fatty acid.
    The transport of fatty acid was poorly understood.
  • essential amino acid
    by specific transporters.
    non-essential fatty acids are blocked.
  • proteins
    insulin: transcytosis

Hypoglycemia:

Glycine is a major inhibitory neurotransmitter in the spinal cord.
$3^ {_ -}P^ {_ -}glycerate \to Serine \to Glycine$

• $3^ {_ -}P^ {_ -}glycerate +NAD^ +\to NADH + 3^ {_ -}phosphohydroxy\ pyruvate$
• $3^ {_ -}phosphohydroxy\ pyruvate + Glut. \to 3^ {_ -}P^ {_ -}serine + \alpha^ {_ -} kg$
• $3^ {_ -}P^ {_ -}serine + PO_ 4 \to Serine$
• $Serine + THF \to Glycine + 5,10^ {_ -}methyl^ {_ -}THF + H_ 2O $

Glutamate is a critical excitatory neurotransmitter in circuits for memory and learning.
$\gamma^ {_ -}aminobutyric\ acid$ (GABA) is the major inhibitory neurotransmitter in many neuronal circuits.

• $\alpha^ {_ -}ketoglutarate + NH_ 3 + NADH \to NAD^ + + Glutamate$ (In glial cells)
• $Glutamate \to Glutamine$ (In glial cells)
• $Glutamine \to Glutamine$ (back to neurons)
• $Glutamate + CO_ 2 \to CO_ 2 + \gamma^ {_ -}aminobutyric\ acid$ (in neurons)

Catecholamine synthesis

Catecholamine synthesis

• $Tyrosine + O_ 2 + BH_ 4 \to BH_ 2 + Dopa$
• $Dopa + CO_ 2 \to Dopamine$
• In some specific neurons
  $Dopamine + O_ 2 + Ascorbate \to Norepinephrine + H_ 2O + Dehydroascorbate$
  $Norepinephrine + CH_ 3 \to Epinephrine$
  Norepinephrine acts both as a hormone, by increasing blood pressure or being part of the fight or flight response (increase alertness)
  Epinephrine is a hormone that has an important function in homeostasis.

Transport of neurotransmitters

Transport protein in the membrane of storage vesicle
V-ATPase: H⁺ Pump
VMAT2: NT in, H⁺ out

Neurons: high energy demands

• Synthesize many neurotransmitters
• Require energy and O₂
  The brain also requires many lipids
• For vesicle turnover
• To form the myelin sheath
• FA synthesis also has high energy demands.

Metabolism Disorder: Phenylketonuria (PKU)

One gene-one protein disorder disease.

Mentally retarded Children with peculiar musty odor urine.

Ferric chloride test:

• Diabetic urine: turn reddish
  target: acetoacetic acid
  -PUK patients: Dark Green
  target: phenylpyruvate

He tests 420 mentally impaired patients:

• 8 had elevated phenylalanine
• 2% of the population in the mental hospital compared to 0.01% in the general population.

Healthy population:

• $phenylalanine \overset{PHA+BH_ 4}{\longrightarrow} tyrosine$
• PAH (phenylalanine hydroxylase) BH₄ is the cofactor of PAH

PKU patients
Inactivation of PAH makes phenylalanine remain at a toxic level among blood and tissues.

Untreated children:

• < 6 months: normal
• 6-12 months: lethargic

• 12 months:

  • Seizures
  • Severe mental retardation
  • Severe Eczema
  • Microcephaly

Phenylalanine in blood

	Healthy	Mild(HPA)	Mild PKU	PKU (uM)
Concentration	<20	120-600	360-1200	>1200

It is passed down generations in an autosomal recessive pattern.

Treatment

Most causes are the miss-fold of the protein.

• The PKU Diet
  The treatment is limited. The most popular measure is taking the PKU Diet which has limited phenylalanine.
• Kuvan (sapropterin dihydrochloride)
  It is a form of BH₄. It works for some of the patients. But the PKU diet is still needed.

PAH

PAH belongs to a family of biopterin aromatic amino acid-dependent hydroxylases that convert phenylalanine, tyrosine, and tryptophan into tyrosine, L-DOPA, and oxitriptan respectively.

The classic PKU is caused by the malfunction of PHA.
The mild PKU is caused by the regeneration of BH₄:

• $BH_ 2 \overset{PCD+DHPR}{\longrightarrow} BH_ 4$

Monomer PAH has three domains:

• Catalytic domain (73% of mutation)
• Self-association domain (6% of mutation)
• The regulatory domain (21% of mutation)
  • allosteric mechanism

Active form PAH is Tetramer

• Allosteric regulation
  Closed E_i form turn to E_a form when the regulatory domain was activated. Both regulatory domain and catalytic domain have a binding site of phenylalanine.
• BH₄
  BH₄ is the coenzyme of PAH. It also binds the regulatory site and helps PAH to stay in an inactive form.
• Phosphorylation
  phosphorylating the serine residue occupying position 16 in the regulatory domain.
  PAH becomes active at a lower concentration of free phenylalanine.

Treatment

• Limited intakes of protein-rich food like meat and potatoes
• Persisting neurological or psychosocial issues and poor quality of life despite early intervention
• Nutritional deficiencies due to diet
• The financial burden due to the cost of special medical food and dietary supplements.

Phenylalanine in excess in the blood acts as a competitive inhibitor, limiting the transport of tyrosine and tryptophan in the brain.

• GMP
  GMP are proteins derived from whey, the residual liquid left after curd formation which is low in phenylalanine.

neurological complication
Despite the restricted diet, some patients still developed neurological complications because they have problems with BH₄ biosynthesis or regeneration.
60% of them affect the enzymes PCD and DHPR.
Among these patients, the administration of a daily dose of 20 mg of BH₄ per kg body weight could improve this situation.

BH₄ acts as a PAH-specific chaperone to leading the correct folding.

PKU with mental-disorder

The accumulation of phenylalanine:

• Oxidative stress
  • Free radicals - form when oxygen interacts with certain molecules
    • Results in highly reactive atoms with an odd number of electrons
    • Has “free” electrons available for pairing
  • Antioxidants
    • Compounds that can “donate” an electron
    • Neutralize oxidative effects
• Neurotransmitters
• Lipid Metabolism
• Bioenergetics
• Calcium

ROS, reactive oxygen species

ROS is the most important group of free radicals in the biology system.

• nature product of cellular metabolism
• low concentration
• Maintained by Antioxidants
• High concentration causes:
  • oxidative stress.
  • Cell components such as DNA, proteins, and cell membranes
    are damaged
• Relationship with PKU
  • PKU diet reduced the injection of antioxidants $\downarrow$
  • PKU diet also limited the synthesis of antioxidants $\downarrow$
  • Increased ROS production by chronic exposure to Phe and metabolites
  • Phe inhibition of the synthesis of endogenous antioxidants

Oxidative Stress in Brain

Neurotransmitter

• Precursors

  • Tyrosine is a precursor of neurotransmitters catecholamines.
  • Tryptophan is a precursor of the neurotransmitter serotonin.

• Inhibitors

  • Natural inhibitors of the enzymes responsible for the syntheses of L-DOPA and dopamine, 5-hydroxytryptophan, and serotonin.

• Dopamine Deficit

  • Parkinson - like symptoms
  • Anhedonia
  • Depression

• Serotonin Deficit

  • OCD - like symptoms
  • Impulsivity
  • Depression

Fatty acid

DHA-Ethylester in brain

• The most abundant polyunsaturated $\Omega$-3 fatty acid
• Responsible for many functions
• Low levels of DHA correlated to severely depressed patients
• Low levels of DHA lower the activity of neuronal activity and increase cell death
• A component of the myelin sheath (Damaged:)
  • Cognitive disruption
  • Neuropathies
  • Difficulty controlling movements and balance

Energy

creatine/phosphocreatine system:
The key enzyme, creatine kinase, was inhibited by phenylalanine.

$Phosphoenolpyruvate +ADP \overset{Pyruvate\ kinase}{\longrightarrow} Pyruvate + ATP$
The key enzyme, Pyruvate kinase, was inhibited by phenylalanine.

The energy production restrictions largely affect the red blood cell and a high level of glycolysis damages the red blood cell structure integrity and are actively phagocytosed. Anemia observed.

Ketone Bodies

• Under normal fasting condition, blood sugar level is low
  • The citric acid cycle slows down
  • Liver cells produce ketone bodies
    • The main source of fuel for the brain
• Synthesis of ketone bodies is decreased by high levels of Phe
  • PKU patients can’t produce as much
  • Decrease in ATP synthesis in the brain

Calcium

High concentration of phenylalanine activates PMCA: increase in calcium concentration efflux (out). Plasmid Ca²⁺ is low

Long term of phenylalanine exposure:
chronic effects: PMCA was inactivated, Plasmid Ca²⁺ is high,
the triggering of action potential is affected.

digraph{ subgraph cluster_0{ color = "pink" style = filled label="Hypoglycemia" fontsize = 20 node [ shape="box"] GLUT1 [label = "GLUT1 initially compensate"] MCT1 [label = "MCT1 Upregulation"] NT [label = "NT synthesis decreases"] "low [Glucose]" -> GLUT1 [label="stimulate"] GLUT1 -> MCT1 [label="failed to compensate"] } subgraph cluster_1{ label="Hypoxia" color = "skyblue" style = filled fontsize = 20 node [ shape="box"] "Increase in fermentation" -> "Accumulation of NADH₁ FADH₂" -> NT } MCT1 -> NT [label="still in energy deficiency"] NT -> "Dangerously low fuel levels" -> "Severe brain damage" }digraph{ Tyrosine2 [label = "Tyrosine"] node [shape=box] Diet -> Tyrosine subgraph cluster_0{ label = Liver fontsize = 20 color = "pink" style = filled Phenylalanine -> Tyrosine2 } Tyrosine2 -> Tyrosine subgraph cluster_1{ label = "Catecholamine synthesis" fontsize = 20 color = "green" style = filled Tyrosine -> Dopamine -> Norepinephrine -> Epinephrine } }digraph{ node [shape=box] BOS [label ="Brain Oxidative Stress", color="skyblue", style="filled"] NCD [label = "Neuronal Cell Death and Brain Aging", color="pink", style="filled"] BOS -> "Astrocytes are impaired\n(Signal transduction)" -> NCD BOS -> "Inflammation" -> NCD BOS -> "imbalance in superoxide level" -> NCD }