Biochem chapter 18: Amino acids

About ____ of energy needs of carnivores can be met by ____ immediately after a meal.

80%; amino acids

Do herbivores and plans use amino acids for energy?

herbivores: only a small fraction

plants: no

How are dietary proteins enzymatically hydrolyzed into amino acids?

• pepsin: proteins→ peptides in the stomach

• trypsin and chymotrypsin: proteins and larger peptides→ smaller peptides

• aminopeptidase and carboxypeptidases A and B: peptides→ amino acids in the small intestine

Once an amino acid is broken down/catabolized, what happens?

1. recycled into new proteins

2. oxidized for energy (urea cycle, glycolysis, citric acid cycle)

What do plants do with nitrogen?

conserve almost all of it

Many aquatic vertebrae’s release ____ to their environment?

ammonia

Many terrestrial vertebrates and sharks excrete nitrogenin the form of what?

urea

• less toxic than ammonia

• very high solubility

Some animals such as birds and reptiles excrete nitrogen as what?

uretic acid

Humans and great apes excrete what?

both urea (from amino acids) and uric acid (from purines)

Describe step 1 of the urea cycle:

removal of the amino group

• ammonia is captured by a series of transamination

• transamination allow the transfer of an amine to a common metabolite like alpha-ketoglutarate to generate glutamate

How is ammonia safety transported in the bloodstream?

glutamine

• excess is processed in the intestines, kidneys, and livers (liver=primary)

What enzyme accepts amino groups?

alpha-ketoglutarate

what enzyme acts as a temporary storage of nitrogen?

L-glutamine

What functional group does pyridoxal phosphate (PLP) contain?

aldehyde group

• aldehyde form can react reversibly with amino groups

What functional group does pyridoxamine phosphate contain?

amine group

• aminated form can react reversibly with carbonyl groups

Describe the pyridoxal phosphate (PLP):

covalently linked to the enzyme in the resting enzyme

• catalyzes multiple reactions

• linkage is made via nucleophilic attack of the amino group of an active-site lysine

What reactions does PLP help to catalyze?

Racemization (mirror images) of amino acids

decarboxylation of amino acids

The ammonia collected in glutamate is removed using what?

glutamate dehydrogenase

• that ammonia is processed into urea for excretion

what is the glucose-alanine cycle?

A way to replenish glucose

• glucose yields pyruvate but if it builds up, then lactic acid will build up

• that excess pyruvate can be converted to alanine to be transported into the liver

• safe transport of nitrogen

Ammonia is recaptured via_______

synthesis of carbamoyl phosphate

• 1st nitrogen-acquiring rection of the urea cycle

What is used in the urea cycle thats not used in the others?

ATP

During the second nitrogen-acquiring reaction, what enters the cycle?

Aspartate

describe how the urea cycle is regulated:

• enzyme carbamoyl phosphate synthetase l

  • activated by N-acetylglutamate

    • formed when glutamate and acetyl-CoA concentrations are high

    • activated by arginine

an increase of urea cycle enzymes when what happens to the body?

high-protein diet (lots of amino acids)

starvation (proteins broken down for energy)

What is the difference between essential and nonessential amino acids?

• essential: obtained as dietary protein

• nonessential: easily made from central metabolites

What are some cofactors that are involved in amino acie catabolism?

• Tetrahydrofolate (THF)

• S-adenosylmethionine (adoMet)

• biotin

*Involved in 1 carbon transfer reactions

Which 1-carbon transfer enzyme is the most versatile?

Tetrahydrofolate (THF)

• can transfer CH3,CH2OH, and CHO

Which 1-carbon transfer enzyme is best at transferring CH3?

S-adenosylmethionine (adoMet)

• synthesized from ATP and methione

• methyl is 1000 times more reactive than THF