Biosynthesis Also known as anabolism Construction of complex molecules from simple precursors Energy...

Biosynthesis

Also known as anabolism

Construction of complex molecules from simple precursors

Energy derived from catabolism used in biosynthesis

Sulfur assimilation

Sulfur is required for the formation of cysteine, methionine and many cofactors

Sulfate (SO42) is often used as a source of sulfur

Sulfate must be reduced before assimilation assimilatory sulfate reduction

Sulfur assimilation

Sulfate is activated by the formation of phosphoadenosine-5-phosphosulfate

Sulfate is then reduced to sulfite (SO3

2) then to hydrogen sulfide (H2S)

Sulfur assimilation

Cysteine is then formed from H2S and used in the formation of other sulfur containing molecules

Nitrogen assimilation

Nitrogen required for proteins, nucleic acids and other important cell constituents

Most microorganisms are incapable of using nitrogen gas as a nitrogen source

They must therefore incorporate either ammonia (NH3) or nitrate (NO3

)

Ammonia incorporation

Ammonia is easily incorporated because it is more highly reduced than other forms of nitrogen

Can be combined with pyruvate to form alanine or -ketoglutarate to form glutamate

Ammonia incorporation

Ammonia can also be incorporated using two enzymes acting in sequence

Glutamine synthetase and glutamate synthetase

Ammonia incorporation

Ammonia used to synthesize glutamine from glutamate

Amino group of glutamine transferred to -ketoglutarate to form 2 molecules of glutamate

Amino group can then be transferred to form other amino acids

Assimilatory nitrate reduction

Nitrate must be converted to ammonia before incorporation into organic compounds

Nitrate is first reduced to nitrite by nitrate reductase

Assimilatory nitrate reduction

Nitrite is reduced to ammonia by nitrite reductase

Ammonia is then incorporated into organic material

Nitrogen fixation

The reduction of gaseous nitrogen to ammonia

Rate of this process often limits plant growth

Carried out by a small number of microorganisms

Nitrogen fixation

Reduction of nitrogen to ammonia is catalyzed by nitrogenase

Sequential addition of electron pairs results in formation of 2 molecules of ammonia from 1 molecule of N2

Nitrogen fixation

Energetically expensive: requires 8 electrons and 16 ATPs

Synthesis of amino acids

Carbon skeletons derived from acetyl-CoA and intermediates of glycolysis, the TCA cycle and the pentose phosphate pathway

Synthesis of amino acids

Synthesis of amino acids

Common intermediates are used to synthesize families of related amino acids

Synthesis of amino acids

Common intermediates are used to synthesize families of related amino acids

Anapleurotic reactions

TCA cycle intermediates used for biosynthesis could be depleted

Anapleurotic reactions serve to replenish cycle intermediates

Anapleurotic reactions

Most microorganisms replace TCA cycle intermediates by CO2 fixation

Different from autotrophs since only used to replace intermediates

Pyruvate or PEP used as acceptor molecule to form oxaloacetate

Glyoxylate pathway

Some microorganisms can use acetate as the sole carbon source

Synthesize TCA cycle intermediates using the glyoxylate pathway

Modified TCA cycle

Glyoxylate pathway

Isocitrate converted to succinate and glyoxylate

Glyoxylate combines with acetyl-CoA to form oxaloacetate

Prevents loss of acetyl-CoA carbons as CO2

Synthesis of purines and pyrimidines

Cyclic nitrogen containing bases that are used in the synthesis of ATP, DNA, RNA and other cell components

Purines contain two joined rings: adenine and guanine

Pyrimidines have a single ring: cytosine, thymine and uracil

Synthesis of purines and pyrimidines

Purine or pyrimidine joined to pentose sugar (ribose or deoxyribose) = nucleoside

Nucleoside + one or more phosphate group = nucleotide

Synthesis of purines

Seven different molecules contribute parts to final skeleton

Synthesis of purines

Inosinic acid is the first common intermediate

Adenosine and guanosine monophosphates formed

Nucleoside diphosphates and triphosphates formed by phosphate transfers from ATP

Synthesis of pyrimidines

Aspartic acid and carbamoyl phosphate combine

Eventually converted to orotic acid

Ribose then added and decarboxylation results in uridine monophosphate

Synthesis of fatty acids

Uses acetyl-CoA and malonyl-CoA as substrates

Malonyl-CoA formed from acetyl-CoA and CO2

Both are transferred to acyl carrier protein (ACP)

Synthesis of fatty acids

Malonyl-ACP reacts with fatty acyl-ACP to yield CO2 and fatty acyl-ACP + 2 carbons

Followed by 2 reductions and a dehydration

Fatty acyl-ACP then ready to accept another malonyl-ACP

Synthesis of fatty acids vs. -oxidation

Reverse process except uses CoA as carrier rather than ACP

Synthesis of lipids

Dihydroxyacetone phosphate reduced to glycerol 3-P

Glycerol 3-P combines with 2 fatty acids to form phosphatidic acid

Attachment of third fatty acid yields triglyceride

Synthesis of lipids

Phosphatidic acid attached to cytidine diphosphate (carrier)

Reacts with serine to form phosphatidylserine

Decarboxylation yields phosphatidylethanolamine