Oligopeptide Digestion and Kwashiorkor Syndrome

Questions and Answers

What is the typical half-life of short-lived proteins?

Days to weeks

Hours to days

Minutes to hours (correct)

Months to years

Which type of proteins are primarily associated with long half-lives?

Misfolded proteins

Short-lived proteins

Regulatory proteins

Structural proteins (correct)

What is a characteristic of structural proteins with regards to their stability?

They have short half-lives

They cannot be synthesized de novo

They are metabolically stable (correct)

They are metabolically unstable

Which amino acid is mentioned in relation to the breakdown estimation of muscle proteins?

Histidine Signup and view all the answers

What is the significance of 3-methylhistidine in muscle protein breakdown?

It indicates actomyosin breakage Signup and view all the answers

Which of the following describes a characteristic of misfolded proteins?

They are considered short-lived proteins Signup and view all the answers

What effect does methylation have on muscle proteins after their incorporation into actomyosin?

It leads to their degradation Signup and view all the answers

What type of proteins typically have a half-life of months to years?

Structural proteins Signup and view all the answers

What occurs during transamination in amino acid catabolism?

An amino group is transferred from an amino acid to an α-keto acid. Signup and view all the answers

Which condition is characterized by increased plasma levels where ALT is less than AST?

Myocardial infarction. Signup and view all the answers

Where does oxidative deamination primarily occur within the cell?

Mitochondrial matrix. Signup and view all the answers

What is released as the result of oxidative deamination?

Ammonia. Signup and view all the answers

What role does nonhepatic disease play in the context of amino acids?

It is associated with lower plasma levels of ALT than AST. Signup and view all the answers

What role do aminopeptidases play in the digestion of oligopeptides?

They hydrolyze oligopeptides at the luminal surface of the intestines. Signup and view all the answers

How are dipeptides and tripeptides further processed after absorption by intestinal epithelial cells?

They are hydrolyzed to free amino acids within the cells. Signup and view all the answers

What happens to excessive amino acids in the body?

They are converted primarily to fatty acids or glucose. Signup and view all the answers

Why are proteins unable to be absorbed directly by the intestines?

Proteins are too large for intestinal absorption. Signup and view all the answers

What pH level does gastric acid achieve, and how does this affect protein digestion?

pH 2-3, too dilute to hydrolyze proteins effectively. Signup and view all the answers

Which substances are primarily involved in amino acid and dipeptide absorption into intestinal epithelial cells?

Dipeptides and free amino acids. Signup and view all the answers

What is a consequence of diets high in protein regarding amino acid metabolism?

Conversion of excess amino acids into glucose or fats. Signup and view all the answers

What are the primary organs involved in the production of proteolytic enzymes for protein digestion?

Stomach, pancreas, and small intestines. Signup and view all the answers

What role does glutamate play in the regeneration of ketoglutarate?

It undergoes rapid oxidative deamination catalyzed by glutamate dehydrogenase. Signup and view all the answers

Which statement about coenzymes in glutamate dehydrogenase reactions is correct?

NADP+ is used in reductive amination. Signup and view all the answers

What activates glutamate dehydrogenase under low energy conditions?

Elevated levels of ADP Signup and view all the answers

Which mechanism is primarily used by muscles for ammonia transport to the liver?

Transamination of pyruvate to alanine Signup and view all the answers

What inhibits the activity of glutamate dehydrogenase?

Elevated concentrations of ATP Signup and view all the answers

Which enzyme is responsible for oxidative deamination of D-amino acids?

D-amino acid oxidase Signup and view all the answers

What is the primary cofactor involved in direct deamination by dehydratases?

Pyridoxal phosphate Signup and view all the answers

Which of the following conditions would favor the synthesis of ketoacids from glutamate?

Low levels of ammonia Signup and view all the answers

What is the effect of high energy molecules like GTP on the reversible reactions controlled by glutamate dehydrogenase?

They inhibit the enzyme. Signup and view all the answers

Which statement best describes the role of amino acid oxidases in amino acid deamination?

They catalyze oxidative deamination of both L- and D-amino acids. Signup and view all the answers

What is the primary function of pepsin in the digestive system?

To denature proteins and break them down into peptides Signup and view all the answers

Which amino acids have the highest size of gradient during active transport?

Glutamate and Glutamine Signup and view all the answers

How is the contraluminal transport of amino acids characterized?

It is Na+-independent Signup and view all the answers

Which of the following is the COAL system responsible for transporting?

Cystine and dibasic amino acids like ornithine, arginine, and lysine Signup and view all the answers

What is the energy source that drives active transport in the context of amino acid movement?

ATP hydrolysis Signup and view all the answers

What is the role of pepsinogen in the stomach?

It is an inactive precursor that is activated to pepsin Signup and view all the answers

Which type of transport system is associated with Na+-dependent luminal transport?

Secondary active transport Signup and view all the answers

What is the major product of peptic hydrolysis of proteins?

Large polypeptides and some free amino acids Signup and view all the answers

Study Notes

Digestive Enzymes and Protein Stability

Cytosolic proteins show varied reactivity based on amino acid terminal structure.
Short-lived proteins, such as regulatory and misfolded proteins, have half-lives of minutes to hours.
Long-lived proteins (e.g., most cellular proteins) have half-lives of days to weeks.
Structural proteins, like collagen, are metabolically stable with half-lives of months to years.
Muscle protein breakdown involves the release of 3-methylhistidine, which is excreted in urine.

Protein Digestion and Absorption

Dietary proteins are too large for intestinal absorption; they must be hydrolyzed into amino acids.
Proteolytic enzymes from the stomach, pancreas, and small intestines facilitate this process.
Gastric juice (including hydrochloric acid and pepsin) denatures proteins and initiates digestion.
Pepsin, an acid-stable endopeptidase, breaks down proteins into large polypeptides and free amino acids.

Enzymatic Digestion in the Small Intestine

Aminopeptidases and dipeptidases at the intestinal lumen cleave amino acids from oligopeptides.
Dipeptides and tripeptides are absorbed and further digested to free amino acids in intestinal epithelial cells.
Free amino acids and dipeptides are transported to the liver via the portal system.

Amino Acid Transport Mechanisms

Active transport systems, driven by ATP hydrolysis, pump amino acids into cells against a concentration gradient.
A significant gradient exists for glutamate and glutamine uptake.
The COAL system facilitates the uptake of cystine and dibasic amino acids (ornithine, arginine, lysine).
Transport mechanisms include Na+-dependent (luminal) and Na+-independent (contraluminal) pathways.

Nitrogen Disposal and Amino Acid Catabolism

Nonhepatic diseases lead to increased plasma levels of transaminases, indicative of tissue damage.
Transamination transfers amino groups to glutamate, the first step in amino acid catabolism.
Oxidative deamination liberates ammonia as a byproduct, primarily in the liver and kidneys.
Glutamate dehydrogenase plays a key role in regenerating α-ketoglutarate and catalyzing oxidative deamination.

Role of Coenzymes and Regulators

Oxidized forms of NAD (NAD+) and NADP (NADP+) serve as coenzymes in deamination reactions.
The normal NADPH/NADP+ ratio in liver is high, supporting various metabolic pathways.
Reaction direction relies on concentrations of glutamate, α-ketoglutarate, ammonia, and oxidized/reduced coenzyme ratios.
Allosteric regulation of glutamate dehydrogenase occurs with ATP, GTP, and NADH acting as inhibitors, while ADP and GDP serve as activators.

Alternative Deamination Mechanisms

L- and D-amino acid oxidases facilitate oxidative deamination, particularly in kidneys and liver.
D-amino acid oxidase, a FAD-dependent enzyme, catalyzes the deamination of D-amino acids, creating ketoacids.
Direct deamination can occur through dehydratases like serine and threonine dehydratases, utilizing pyridoxal phosphate.

Ammonia Transport to the Liver

Two primary mechanisms transport ammonia to the liver:
- First Mechanism: Involves glutamine synthetase and glutaminase, prevalent in most tissues.
- Second Mechanism: Operates primarily in muscles and involves the transamination of pyruvate to alanine.

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Description

Explore the digestion of oligopeptides by the enzymes located in the small intestines, including aminopeptidases and dipeptidases. This quiz also covers the consequences of high protein diets and the metabolic pathways of excess amino acids. Test your knowledge on these essential biochemical processes!