Lecture 16

Question 1

Starting with uptake, how do most plants/ bacteria generate a usable form of nitrogen? Which enzymes are invovled?

Answer 1

They take up nitrate and convert it back to nitrite (nitrate reductase), when then converts to ammonia (nitrite reductase), then Gln (Gln synthetase).

Question 2

Reactants of tryptophan synthesis?

Answer 2

Chorismate and glutamine. Gives off pyruvate and glutamate.

Question 3

Reactants in last step of tryptophan synthesis?

Answer 3

Input serine and PLP cofactor to indole-3-glycerol phosphate. Release water and glyeraldehyde 3-P. Makes tryptophan

Question 4

Describe channeling in tryptophan biosynthesis

Answer 4

Trp synthase has a 25 A° long channel connecting the active sites. The diameter is the indole diameter and the tunnel helps the indole intermediate diffuse through and prevents escaping to solvent

Question 5

Explain the Malliard reaction in french fries

Answer 5

Asparagine + heat + sugars = acrylamide, a carcinogen

Question 6

Describe the first step in dietary protein digestion

Answer 6

The mouth has no proteases, so the stomach is the first step. Gastrin signals parietal cells to secrete HC, while chief cells secrete pepsinogen. Pepsinogen gets cleaved into pepsin, which cuts proteins into peptides

Question 7

Describe the second step in dietary protein digestion

Answer 7

The intestines cleave trypsinogen, chymotrypsinogen, and aminopeptidases/carboxypeptidases into their active forms

Question 8

Difference between chymotrysin/trypsin and aminopeptdiases/carboxypeptidases?

Answer 8

Former cuts proteins/large proteins into smaller peptides, but the latter degrades peptides into single amino acids

Question 9

What are the two mechanisms for protein degradation?

Answer 9

Ubiquitin/proteosome system (UPS) and autophagy

Question 10

Describe the UPS in cellular protein breakdown

Answer 10

The UPS degrades single proteins. Ub ligase adds the ubiquitin degradation tag to the protein, then the proteosome binds the protein and degrades it

Question 11

Describe autophagy

Answer 11

The autophagy receptor binds Ub on the protein AGGREGATE and the autophagosome traps the protein aggregate for degradation

Question 12

What is the product of catabolizing muscle protein?

Answer 12

Alanine

Question 13

Describe the first step in muscle protein breakdown

Answer 13

Muscle protein breaks down into amino acids

Question 14

Second step in muscle protein breakdown

Answer 14

Amino acids convert to glutamate

Question 15

Third step in muscle protein breakdown

Answer 15

Glutamate goes through alanine aminotransferase to make alanine and alpha-ketoglutarate

Question 16

Fourth step in muscle protein breakdown

Answer 16

Alanine goes from muscle to the blood, then to the liver

Question 17

Fifth step in muscle protein breakdown

Answer 17

Alanine reacts with alpha-ketoglutarate through alanine aminotransferase to make glutamate, which can then make ammonia for the urea cycle OR alanine can form pyruvate for gluconeogenesis (which can regenerate alanine)

Question 18

Where does the urea cycle occur?

Answer 18

Liver

Question 19

Describe the two pathways for amino acid catabolism

Answer 19

Urea cycle: convert aa to NH3 -> carbamoyl phoosphate -> urea cycle

TCA cycle: aa -> carbon skeleton -> keto acids -> TCA cycle

Question 20

Which pathway connects the urea cycle and TCA cycle?

Answer 20

Asp-Arg shunt

Question 21

Which 4 amino acids play the main roles in nitrogen metabolism?

Answer 21

Glu, Gln, Ala, and Asp

Question 22

Which TCA cycle amino intermediates are the main amino acids easily converted to?

Answer 22

Glu and Gln to alpha-ketoglutarate, alanine to pyruvate, and Asp to OAA