chapter 18- amino acid oxidation and production of urea

Alanine Transports Ammonia from Skeletal Muscles to the Liver -What kind of form does alanine play an important role in transporting amino groups to liver in? ---What pathway does this occur via? --- In what form are amino groups collected in muscle and other tissues that degrade amino acids for fuel? By what process? ---What can glutamate also be converted to? For the transport to where? Or it can transfer its a-amino group to what? By action of what?

-Alanine also plays an important role in transporting amino groups to the liver in a nontoxic form ---This occurs via a pathway called the glucose-alanine cycle ---In muscle and certain other tissues that degrade amino acids for fuel, amino groups are collected in the form of glutamate by transamination ---Glutamate can also be converted to glutamine for transport to the liver or can transfer its a-amino group to pyruvate by action of alanine aminotransferase

Glutamine Transports Ammonia in the Bloodstream ***************************************** -What central role does glutamine play in all organisms? What does it serve as in microorganisms? -Is glutamine in excess in most terrestrial animals?

-Glutamine also serves as a source of amino groups in a variety of biosynthetic reactions. Glutamine synthetase is found in all organisms, always playing a central metabolic role. In microorganisms, the enzyme serves as an essential portal for the entry of nitrogen into biological systems. -In most terrestrial animals, glutamine in excess of that required for biosynthesis is transported in the blood to the intestine, liver, and kidneys for processing. In these tissues, the amide nitrogen is released as ammonium ion in the mitochondria, where the enzyme glutaminase converts glutamine to glutamate and NH4+. The NH4+ from intestine and kidneys is transported in the blood to the liver. In the liver, the ammonia from all sources is disposed of by urea synthesis.

metabolic fates of amino groups: xtra notes

-Many aquatic vertebrates release ammonia to their environment. ---passive diffusion from epithelial cells ---active transport via gills -Many terrestrial vertebrates and sharks excrete nitrogen in the form of urea. ---Urea is far less toxic that ammonia. --Urea has very high solubility. -Some animals such as birds and reptiles excrete nitrogen as uric acid. ---Uric acid is rather insoluble. ---Excretion as paste allows the animals to conserve water. -Humans and great apes excrete both urea (from amino acids) and uric acid (from purines).

dietary protein is enzymatically degraded to amino acids notes

-The acidic gastric juice (pH 1.0 to 2.5) is both an antiseptic, killing most bacteria and other cells, and a denaturing agent, unfolding globular proteins and rendering their internal peptide bonds more accessible to enzymatic hydrolysis. -The parietal cells and chief cells of the gastric glands secrete their products in response to the hormone gastrin. -Pepsin begins the process of protein degradation in the stomach. -pepsin chews up protein and hydrolyzes it to small oligopeptides

Glutamate Releases Its Amino Group as Ammonia in the Liver Where are the amino groups from many of the AA collected in? In the form of the amino group of what molecules?

-The amino group from many of the a-amino acids are collected in the liver in the liver in the form of the amino group of L-glutamate molecules

Alanine Transports Ammonia from Skeletal Muscles to the Liver -What can glutamate be converted to for transport to the liver? -Or what kind of transfer can happen by action of alanine aminotransferase? -Where does the alanine formed pass into and then travel into? -In the cytosol of hepatocytes, alanine aminotransferase transfers the amino group from alanine to a-ketoglutarate, to form what? -Glutamate can then enter what, where the glutamate dehydrogenase reaction releases NH4+. (or undergo transamination with oxaloacetate to form aspartate)

-glutamine -glutamate can transfer its a-amino group to pyruvate by action of alanine aminotransferase. -alanine aminotransferase transfers the amino group from alanine to a-ketoglutarate to form pyruvate and glutamate -glutamate can enter mitochondria

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate What do all aminotransferases have? -What is the prosthetic group? The coenzyme form of what? -What does PLP function as? At the active site of what? -What is PLP generally covalently bound to? Through what kind of linkage? To what group of what AA residue?

-same prosthetic group and same reaction mechanism -prosthetic group: PLP (pyridoxal phosphate), the coenzyme form of pyridoxine (Vitamin B6) -PLP functions as an intermediate carrier of amino groups at the active site of aminotransferases -PLP is generally bound to the enzyme's active site through a Schiff base linkage to the e-amino acid group of a Lys residue

Glutamine Transports Ammonia in the Bloodstream -Is ammonia toxic to animal tissues? Are the levels present in blood tightly controlled? -In most animals, what is the free ammonia converted? Exported from ________ tissue into ________ and transported to _______ or ____________. -For transport function, what is glutamate converted to? -What is the free ammonia produced in tissues combined with? To yield what? By action of? -What is the nontoxic form of ammonia? Is it normally present in blood in much higher concentrations than other amino acids?

-yes -In most animals, much of the free ammonia is converted to a nontoxic compound before export from the extrahepatic tissues into the blood and transport to the liver or kidneys -For this transport function, glutamate is converted to L-glutamine -The free ammonia produced in tissues is combined with glutamate to yield glutamine by the action of glutamine synthetase -Glutamine is a nontoxic form of ammonia; it is normally present in the blood in much higher concentrations than other amino acids

intro to amino acid oxidation In animals, how to amino acids undergo oxidative degradation in 3 different metabolic circumstances?

1. normal synthesis and degradation of cellular proteins 2. When a diet is rich in protein and the ingested amino acids >> body's needs for protein synthesis, surplus is catabolized (amino acids cannot be stored) 3. During starvation or in uncontrolled diabetes mellitus, when carbohydrates are either unavailable or not properly utilized, cellular proteins are used as fuel

Where does alanine transport ammonia from? To what?

Alanine Transports Ammonia from Skeletal Muscles to the Liver

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate What kind of reactions are amino transferases a classic example of enzymes catalyzing? What happens?

Amino transferases are classic examples of enzymes catalyzing bimolecular Ping-Pong reactions, in which the first substrate reacts and the product must leave the active site before the second substrate can bind. Thus, the incoming amino acid binds to the active site, donates its amino group to pyridoxal phosphate, and departs in the form of an alpha-keto acid. The incoming a-keto acid then binds, accepts the amino group from pyridoxamine phosphate, and departs in the form of an amino acid.

intro to amino acid oxidation How do carnivores compare to herbivores for the contribution of energy from amino acids?

Carnivores consume primarily protein and thus must obtain most of their energy from amino acids, whereas herbivores may fill only a small fraction of their energy needs by this route

dietary protein is enzymatically degraded to amino acids What does the entry of dietary protein into the stomach stimulate? To secrete which hormone? Which stimulates the secretion of what? By which cells? And stimulates the secretion of what? By which glands?

Entry of dietary protein into the stomach stimulates the gastric mucosa to secrete the hormone gastrin, which in turn stimulates the secretion of hydrochloric acid by the parietal cells and pepsinogen by the gastric glands

metabolic fates of amino groups What happens to excess ammonia generated in extra-hepatic tissues?

Excess ammonia generated in extrahepatic tissues travels to the liver for conversion to the excretory form

metabolic fates of amino acids Excess ammonia generated in most other tissues is converted to the amide nitrogen of , which passes to the , then into liver

Excess ammonia generated in most other tissues is converted to the amide nitrogen of glutamine, which passes to the liver, then into liver mitochondria

What does glutamate release its amino group as? Where?

Glutamate Releases Its Amino Group as Ammonia in the Liver

Glutamate Releases Its Amino Group as Ammonia in the Liver How many id subunits in glutamate dehyrogenase? What is its activity influenced by?

Glutamate dehydrogenase is an allosteric enzyme with six identical subunits. Its activity is influenced by a complicated array of allosteric modulators. The best studied of these are the positive modulator ADP and the negative modulator GTP. The metabolic rationale for this regulatory pattern has not been elucidated in detail. Mutations that alter the allosteric binding site for GTP or otherwise cause permanent activation of glutamate dehydrogenase lead to a human genetic disorder called hyperinsulinism-hyperammonemia syndrome (rare disease), characterized by elevated levels of ammonia in the bloodstream and hypoglycemia. Prevalence is estimated at 1 in 200,000.

Glutamate Releases Its Amino Group as Ammonia in the Liver Where is L-glutamate dehydrogenase present in in mammals? What is it the only enzyme that can use as the acceptor of reducing equivalents? What two processes can a-ketoglutarate fromed from glutamate deamination be used in?

In mammals, L-glutamate dehydrogenase is present in the mitochondrial matrix. It is the only enzyme that can use either NAD+ or NADP+ as the acceptor of reducing equivalents. The a-ketoglutarate formed from glutamate deamination can be used in the citric acid cycle and for glucose synthesis.

metabolic fates of amino acids Fill in: In the of liver cells, amino groups from most amino acids are transferred to a-ketoglutarate to form , which enters and gives up its to form .

In the cytosol of liver cells, amino groups from most amino acids are transferred to a-ketoglutarate to form glutamate, which enters mitochondria and gives up its amino group to form NH4+

dietary protein is enzymatically degraded to amino acids In the stomach, what does pepsin hydrolyze? At which bonds? To convert what to what?

In the stomach, pepsin hydrolyzes ingested proteins at peptide bonds to convert longer polypeptide chains into a mixture of smaller peptides

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate notes

PLP = pyridoxal phosphate Cells contain different types of aminotransferases. Many are specific for alpha-ketoglutarate as the amino group acceptor, but differ in their specificity for the L-amino acid. The enzymes are named for the amino group donor (ex. Alanine aminotransferase). The reactions catalyzed by aminotransferases are freely reversible, having an equilibrium constant of approximately 1. -Bottom left is ammonia donor. Glutamate is acceptor. Transferase have specifities for donors ---alanineaminotransferase: alanine is donor -Glutamate is reservoir for ammonia

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate notes

PLP undergoes reversible transformations between its aldehyde form (pyridoxal phosphate), which can accept an amino group, and its aminated form, pyridoxamine phosphate, which can donate its amino group to an alpha-keto acid -PLP is transient carrier of ammonia -pyridoxal phosphate -aldehyde group is where u put ammonia -amine attacking carbonyl a

dietary protein is enzymatically degraded to amino acids What is pepsinogen an inactive precursor to? What is it converted to? By what mechanism? What condition does this only occur in?

Pepsinogen, an inactive precursor zymogen, is converted to active pepsin by an autocatalytic cleavage that occurs only at low pH

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate What kind of base is PLP generally bonded to an enzyme through? aka? What does this activated form of PLP undergo? To form what kind of base? With what group of the substrate amino acid?

Pyridoxal phosphate is generally bonded to the enzyme through a Schiff base, also called an internal aldimine. This activated form of PLP readily undergoes transimination to form a new Schiff base (external aldimine) with the α-amino group of the substrate amino acid (see Fig. 18-5b, d).

dietary protein is enzymatically degraded to amino acids What organ does secretin stimulate? What does it then secrete? Where? To do what to the gastric HCl? What does this do to pH?

Secretin stimulates the pancreas to secrete bicarbonate into the small intestine to neutralize the gastric HCl (abruptly raising the pH to about 7)

dietary protein is enzymatically degraded to amino acids

Synthesis of digestive proteases as inactive precursors protects the exocrine cells from self-destructive proteolytic attack. The pancreas further protects itself against self-digestion by making a specific inhibitor, a protein called pancreatic trypsin inhibitor. Given the key role of trypsin in proteolytic activation pathways, inhibition of trypsin effectively prevents premature production of active proteolytic enzymes within pancreatic cells. Degradation of short peptides in the small intestine is then completed by other intestinal peptidases (carboxypeptidases A and B), which removes successive carboxy-terminal residues from peptides as well as aminopeptidases. In humans, most globular proteins from animal sources are almost completely hydrolyzed to amino acids in the gastrointestinal tract, but some fibrous proteins, such as keratin, are only partly digested. Acute pancreatitis is a disease caused by obstruction of the normal pathway by which pancreatic secretions enter the intestine. The zymogens of the proteolytic enzymes are converted to their catalytically active forms prematurely, inside the pancreatic cells, and attack the pancreatic tissue itself. This causes excruciating pain and damage to the organ that can prove fatal.

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate What does glutamate function as? For which pathways? What does it lead to the elimination of?

The glutamate then functions as an amino group donor for biosynthetic pathways or for excretion pathways that lead to the elimination of nitrogenous waste products

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate Three alternative fates for the external aldimine are shown: A transamination, B racemization, and C decarboxylation. The PLP-amino acid Schiff base is in conjugation with the pyridine ring, an electron sink that permits delocalization of an electron pair to avoid formation of an unstable carbanion on the α carbon (inset). A quinonoid intermediate is involved in all three types of reactions. The transamination route A is especially important in the pathways described in this chapter.

The pathway highlighted in yellow (shown left to right) represents only part of the overall reaction catalyzed by aminotransferases. To complete the process, a second α-keto acid replaces the one that is released, and this is converted to an amino acid in a reversal of the reaction steps (right to left). Pyridoxal phosphate is also involved in certain reactions at the β and γ carbons of some amino acids (not shown).

dietary protein is enzymatically degraded to amino acids Where is the resulting mixture of free amino acids transported to? Through which the amino acids enter what? And travel to where?

The resulting mixture of free amino acids is transported into the epithelial cells lining the small intestine, through which the amino acids enter the blood capillaries and travel to the liver

metabolic fates of amino acids Glutamine/glutamine are important. What do they act as?

a kind of collection point for amino groups

metabolic fates of amino acids What is glutamate/glutamine converted to?

a-ketoglutarate

metabolic fates of amino acids In skeletal muscle, what are excess AA generally transferred to?

alanine to pyruvate

What does glutamine transport in the bloodstream?

ammonia

dietary protein is enzymatically degraded to amino acids Arrival of AA in the upper part of the intestine (duodenum) causes the release into the blood of what hormone? What does this the stimulate the secretion of?

causes release into the blood of the hormone cholecystokinin (stimulates secretion of several pancreatic enzymes with activity optimum at pH 7-8)

metabolic fates of amino acids What can these AA readily convert to molecules to be used in ?

citric acid

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate What is the effect of the transamination reactions?

collect the amino groups from many different amino acids in the form of L-glutamate

metabolic fates of amino groups What is the AA derivation that is the source of most amino groups?

dietary protein

intro to amino acid oxidation derivations of protein?

dietary protein and tissue protein

overview of amino acid oxidation What important feature distinguishes amino acid degradation from other catabolic processes?

every amino acid contains an amino group, and the pathways for amino acid degradation include a key step in which the a-amino group is separated from carbon skeleton

dietary protein is enzymatically degraded to amino acids In humans, where does the degradation of ingested proteins to their constituent amino acids occur?

gastrointestinal tract

metabolic fates of amino acids What four amino acids play central roles in nitrogen metabolism?

glutamate, glutamine, alanin, aspartate

metabolic fates of amino acids What AA are present in higher concentrations than other AA in most tissues?

glutamin or glutamate

metabolic fates of amino groups Where are most amino acids metabolized?

in the liver

metabolic fates of amino groups What element is abundant in atmosphere, but too inert for use in most biochemical processes?

nitrogen, N2

metabolic fates of amino acids What is aspartate converted to?

oxaloacetate

intro to amino acid oxidation How can amino acids make a significant contribution to the generation of metabolic energy?

oxidative degradation

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate Once they have reached the liver, what is the first step in the catabolism of most L-amino acids? What enzyme is it promoted by?

removal of the a-amino groups, promoted by enzymes called aminotransferases or transaminases

Pyridoxal Phosphate Participates in the Transfer of a-Amino Groups to a-Ketoglutarate In these transamination reactions. what is the a-amino group transferred to? What does it leave behind?

the a-amino group is transferred to the a-carbon of a-ketoglutarate, leaving behind the corresponding a-keto acid analog of the amino acid

dietary protein is enzymatically degraded to amino acids As the acidic stomach contents pass into the small intestine, what kind of pH triggers the secretion of a hormone into the blood? What is this hormone?

the low pH triggers secretion of the hormone secretin into the blood

What kind of transfer does pyridoxal phosphate participate in?

transfer of a-amino groups to a-ketoglutarate

dietary protein is enzymatically degraded to amino acids What two things further hydrolyze peptides that were produced by pepsin in the stomach?

trypsin and chymotrypsin

intro to amino acid oxidation Does the fraction of metabolic energy obtained from amino acids vary greatly with the type of organism and metabolic conditions?

yes

overview of amino acid oxidation Do the amino groups and carbon skeleton take separate, but interconnected pathways?

yes

metabolic fates of amino groups What happens to some of the ammonia metabolized from dietary protein?

§ Some of the ammonia generated in this process is recycled and used in a variety of biosynthetic pathways; the excess is either excreted directly or converted to urea or uric acid for excretion, depending on the organism

Glutamate Releases Its Amino Group as Ammonia in the Liver -These amino groups must be removed from glutamate to prepare them for what? -In hepatocytes, where from the cytosol is glutamate transported to? What does it undergo? What is it catalyzed by? -What is the combined action of an aminotransferase and glutamate dehydrogenase referred to?

§ These amino groups must next be removed from glutamate to prepare them for excretion § In hepatocytes, glutamate is transported from the cytosol into the mitochondria, where it undergoes oxidative deamination catalyzed by L-glutamate dehydrogenase § The combined action of an aminotransferase and glutamate dehydrogenase is referred to as transdeamination