Hormones and Neurotransmitters - Biochemistry

Epinephrine and glucagon are examples of hormones that affect G protein-coupled receptors like adenylate cyclase and increase levels of cAMP. The other hormones listed affect receptor tyrosine kinases and non-receptor tyrosine kinases.

Elevated arginine leads to an increase in secretion of insulin, not decreased. GIP, CCK and closing of the voltage gated potassium channels lead to an increase in secretion of insulin.

Ketone bodies, which include acetoacetate, beta-hydroxybutyrate, and acetone, are produced by the liver in the fasting state by beta-oxidation of fatty acids. They are then released into the blood stream, where they can be used as alternative energy sources for other organs, such as muscle, kidney, and brain. Apoprotein B is one of the proteins that hold lipoproteins together. Beta-carotene is a vitamin with antioxidant properties.

Out of all the neurotransmitters listed, the only one that isn't a catecholamine is serotonin. This neurotransmitter is initially derived from the amino acid tryptophan, whereas the catecholamines are derived from the amino acid tyrosine.

Dopamine, norepinephrine, and epinephrine are all catecholamine neurotransmitters. In fact, in the metabolic pathway that produces these compounds, dopamine is an intermediate that can be converted into norepinephrine, which can subsequently be converted into epinephrine.

Tyrosine hydroxylase is the rate-limiting enzyme for catecholamine synthesis. It catalyzes the conversion of tyrosine to dihydroxy-phenylalanine (DOPA). Tryptophan hydroxylase is the rate-limiting step for serotonin synthesis. Dopamine beta-hydroxylase converts dopamine to norepinephrine. Amino acid decarboxylase converted DOPA to dopamine.

Chromaffin cells are located in the adrenal gland, and release epinephrine and norepinephrin.

Chromaffin cells release both epinephrine and norepinephrine, but 80% of the neurotransmitters released is epinephrine.

Epinephrine first binds to an adrenergic receptor. The activated receptor works via a G protein, and so GDP is exchanged for GTP and the protein is activated. This then causes activation of adenylate cyclase and subsequent conversion of ATP to cAMP. cAMP acts upon protein kinase A and several other effector molecules. Diacylglycerol and IP3 are second messengers that are uninvolved in this process.

Of the options given, only norepinephrine is synthesized in storage vesicles. The rest are synthesized in cytoplasm.

Dihydroxyphenylalanine (DOPA) is the precursor for dopamine. Of the options, only dopamine, epinephrine, and norepinephrine are released upon nerve firing.

Erythropoietin is a glycoprotein hormone produced in the kidney that stimulates the production of red blood cells in the bone marrow.

Glucagon is a short peptide hormone involved in triggering signal cascades in response to low blood glucose. Biotin, also known as vitamin B7, is a cofactor in fatty acid synthesis. Pyridoxal phosphate, the activated form of vitamin B6, is a cofactor in transamination reactions, among others. Epinephrine is a steroid hormone involved in the fight or flight response.

Tyrosine hydroxylase is the rate-limiting enzyme for all catecholamine synthesis reactions. L-aromatic amino acid decarboxylase is needed to catalyze the step from DOPA to dopamine. Norepinephrine synthesis requires dopamine beta-hydroxylase and epinephrine synthesis requires dopamine beta-hydroxylase and phenylethanolamine N-methyltransferase (PNMT) in addition to the other enzymes mentioned.

NETs are used to remove norepinephrine from the synaptic cleft. Tryptophan hydroxylase and amino acid decarboxylase are part of the serotonin synthesis pathway. VMA is a breakdown product of norepinephrine.

Parkinson disease is associated with decreased dopamine concentration. It is commonly treated with L-DOPA, which can cross the blood brain barrier and be converted to dopamine.

Tryptophan is the original substrate for serotonin synthesis. All other answers are involved in the catecholamine synthesis pathway.

Tryptophan hydroxylase is the rate-limiting step of serotonin synthesis, not to be confused with tyrosine hydroxylase.

Monoamine oxidase (MAO) is responsible for inactivating catecholamines and serotonin. Thus, inhibiting MAO would result in an increase of catecholamines and serotonin.

Of the options, only COMT is involved with catecholamine and serotonin breakdown. The rest are enzymes in the catecholamine synthesis pathway.